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And this is the first of our deep dives into the 71% of the surface of our planet that we’re biologically designed to be least interested in (and more fool us, as I hope to prove to you multiple times over the next month or so).

Here’s a story about a sea that came back to life in the most dramatic way imaginable.

(You’re going to need your full imagination for this one. It gets REALLY DAMN BIG.)

It’s 2016, and I’m on a ship, fulfilling one of my lifelong dreams.

It’s all down to an article I read as a kid in one of my dad’s many copies of National Geographic - a romantic, stirring portrayal of Gibraltar, that last peaceful haven of ‘civilization’ before you set sail for the wilder thrills and terrors of Truly Foreign Lands. (Sigh. Well, I was only a kid.)

But as I grew up, I learned more and more about Africa: about how unimaginably vast it was compared to everywhere I’d ever been, about its beauty and its ugliness, and its bewildering diversity of peoples and cultures and ideas and landscapes and ways of living and thinking and being…

I also read travel writing about it (mainly from men who were non-Africans, still a huge issue) - and in most of what I read, Africa left that writer changed forever.

I wanted that too. And to get the most amount of change on offer, I decided to arrive in Africa without cheating.

It had to be by sea. No flying allowed. I would arrive in Africa after voyaging between the Pillars of Hercules, across one of the great seaways of the world. 

(Would it be perilous? Maybe! A bit would be nice. Isn’t that what adventurers deliberately seek out, just the right amount of peril? Tally-ho!)

Looking back, a lot of this is troublesome: an entirely selfish dream about what Africa would give me. But still, on readying myself for this sea crossing in 2016, it felt like the right call.

I felt it in Algeciras, looking across the sea to the mountains of Morocco, absurdly close across the Strait. 

I felt it on the ferry, as those mountains gained detail and my thoughts caught up with themselves (“woah, that’s AFRICA…”). 

And I even felt it in Tangier when my girlfriend and I were arguing with taxi-drivers fanatically intent on charging us 20 times the going rate, presumably because I had exactly the kind of facial expression of someone who should never, ever visit anywhere outside of the UK on his own. (There’s your peril, Mike. Enjoy!)

In crossing the Strait of Gibraltar by sea, taking great lungfuls of sea air and snapping away madly with my crappy camera-phone, I thought I was having the best experience available - and inviting into my excitable brain the maximum amount of awe and wonder available to me.

In fact, I was looking for them in entirely the wrong direction.

Six million years ago, the Mediterranean Sea looked very different. 

For starters, “sea” is a misnomer. Because it appears there wasn’t one. In place of the 1,000,000 cubic miles of seawater that currently stretches 2,500 miles (4000 km) from Gibraltar to the Gulf of İskenderun (Turkey), there was…

Something. It’s unclear what. But it wasn’t a “sea”.

A big problem here is how unimaginably distant 6 million years is. Putting it simply, that’s time for a lot of monumentally huge things to occur and then be completely erased by the most gradual of natural processes.

(When I was an archaeologist, we always had to think about the story-muddying effects of taphonomy, the natural processes that break down & scatter biological material. It’s detective work that often gets brain-bendingly complicated - and that’s only across centuries or a couple of thousand years.)

Over millions of years, rainfall can completely dissolve away rock, erosion can strip away vertical kilometres of soil and sediment - and the tectonic plates determining the shape of land & sea can buckle and heave into forms we wouldn’t recognise today. At a local scale, it’s enough time for anything to happen.

For indicators of what actually did happen, we turn to geology.

In the 1960s, a seismic survey (think “geological ultrasound”) of the floor of the Mediterranean uncovered a continuous feature a few hundred metres down. Further investigations by the crew of the research vessel Glomar Challenger in the ‘70s confirmed both its existence and composition.

It was salt.

Lots and lots of salt.

More salt than you’re capable of believing, I’ll bet.

Some of these layers of salt were two miles thick. As National Geographic science writer Maya Wei-Haas puts it: “enough to give each of the world’s 7.7 billion people nearly 50 Great Pyramids of Giza filled with the stuff.”

(Read that again, if you like! It won’t help though. Sorry.)

Salt comes from evaporated seawater - so, salt on this scale? It must have been the mother of all dessications. 

Scientists now refer to this as the Messinian Salinity Crisis, named after the geological age it took place in. In some presumed tectonic upheaval, the sea floor of what’s now the Strait of Gibraltar was thrust upwards, forming a bridge that cut the Mediterranean off from the Atlantic, its main replenishing source of water…and the Med started drying out. 

After some amount of time - certainly much than all recorded human history, probably even longer than the 300,000 years that anatomically modern humans have existed on Earth - the bowl of the Mediterranean was a deep, empty desert, save for a few huge, super-salty lakes of unknown size. (There’s fossil evidence that suggests the presence of marine life during this time, so the Med is unlikely to have completely dried out.)

An enormous salty dryness, up to 900 miles across, with sloping cliffs over a mile deep - shallower in the west of the Mediterranean bowl where the seabed is former of continental crust, much deeper in the east (and at its extreme eastern end, an incredible “Egyptian Grand Canyon,” formed from the foundations of the Nile)… 

And at its Western end, a plug - a strip of land just a few miles wide - with the force of the whole Atlantic behind it.

A few decades ago, Daniel Garcia-Castellanos, a geophysicist at Geosciences Barcelona, grew curious about what happened next. 

How quickly could a whole sea be refilled?

It’s easy to imagine centuries of streams and rivers trickling water back in, nibbling away at that land-bridge, a little stronger every year. A uniformitarian model, as was most popular during the earliest days of the science of geology, based on the kind of gradual change we can see at work today. It’s a no-brainer concept aided by what can see with your own eyes: go for a walk in the mountains, chart the course of a stream cutting its way through solid rock for thousands of years, extrapolate all your models accordingly. What could be more logical? I mean, all this geology didn’t happen overnight, did it?

But the gradualistic model didn’t tally with the work Garcia-Castellanos had previously done around tectonic lakes:  

“In the models we were running in Amsterdam, this transition was very fast.”

It seemed more likely that while a stream or river maybe have taken hundreds or thousands of years to cut back to the Atlantic side and form a breach, when that critical tipping-point was reached, 90% of the floodwater powered through according to a human timescale:

“When we got the first predictions, I was very surprised and thought maybe something was wrong with the formulations,” Garcia-Castellanos later told Nature. “If the model was correct, we would expect to find traces of the flood erosion preserved under the sedimentary layers in the strait.”

So he and his team went looking - and found the marks of a cataclysm.

There on the floor of the Strait of Gibraltar was a massive channel, carved through sediment and rock, hundreds of meters deep and running from the Gulf of Cadiz to the Algerian Basin - a distance of nearly 390 km. If this did indeed happen quickly, and this was the result…can you imagine the violence at work here?

This is a silly question. Nobody can imagine it, even Roland Emmerich, so we have to resort to using numbers.

Have you ever seen the famous Niagara Falls, on the border between Ontario and New York? Or even better, have you felt them, standing there in person, with all that water roaring past, feeling the spray against your face and the thunder of it shivering through your knees?

Every second, around 2,400 cubic metres of water tumble over Niagara Falls, representing a total weight of around 3,100 tons, which pounds the base of the waterfall with a force of 2,509 tons. That’s every second, remember.

(It’s very hard to put any of this into a meaningful human perspective - so I’ll just say that the weight of an Airbus A380, currently the world’s largest passenger airliner, is a touch under 620 tons.)

In contrast, the flood Garcia-Castellanos and his team were modelling seemed to require a peak flow of a hundred million cubic metres of water a second.

This wasn’t a waterfall, but a catastrophic deluge down a gigantic slope extending east from the Strait for hundreds of miles - and perhaps looking a little like this image by palaeoartist Joschua Knüppe…

(Except maybe at a shallower incline, and without the solitary human figure, because we wouldn’t exist for another 5-and-a-bit million years.)

This torrent would be fast enough to refill the Mediterranean not in centuries or even decades, but in years - or even months. A megaflood, dwarfing even the colossal Missoula Floods in modern-day Washington state at the end of the last ice age.

This is the kind of thing we laugh at in disaster movies, because Oh come on, that’s just daft. Get a grip! Deep down, we know the world doesn’t really work like this, which is why we have a fondness for Godzillas and crashing alien motherships and the like.

But…what if it sometimes does happen this dramatically?

(After all, this blood-curdling, Pulitzer-winning piece on Pacific coast earthquakes is based on hard science.)

For understandable reasons, the Zanclean Megaflood model has proved controversial - so for the last decade, Garcia-Castellanos and his fellow researchers have been busily searching for more evidence. Many important questions needed answering. For example: where did all that ripped-up sediment (enough to fill “400 million olympic swimming pools”) from that 390 km-long trench go?

Feast your eyes a second on this beauty: a National Geographic map from 1982 (I remember the exact issue it was in, and remember unfolding the map on the kitchen floor and obsessing over it until my mum rounded a corner and tripped over both of us).

Notice how between the west and east, everything narrows and bunches up, just below Sicily. If the incoming floodwater continued eastwards to fill up the seas around Greece, Turkey and Egypt (and refill the parched, cavernous Nile), you’d think some of that torn-up sediment would be dumped where the way was narrowest?

In 2018, a team led by Professor Aaron Micallef from the University of Malta announced their findings:

This chaotic mess of sediments, fanned out over a huge area with an amphitheatre-shaped head 6 km wide, lay just underneath the Malta Escarpment - a steep, now-underwater cliff running north-south that descends as far as 3km to reach the eastern Mediterreanean sea floor.

The implication? At some point in the last 6 million years, there was one hell of a waterfall here, dumping a stupendous quantity of seawater rich with sediment - and it happened quickly, because it seems this sediment has none of the stratigraphy you see when it’s slowly laid down, sorted by gravity and other factors to create neat, ordered layers in the rock, like an expertly-made cake. (This? This is more like the rubble after a bomb-blast.)

In 2020, Garcia-Castellanos and Aaron Micallef co-authored a paper with their colleagues, which you can read here. Amongst their other compelling findings, it suggests the one-time presence of a waterfall 1.5 km high, over which flood-waters tore at speeds up to 160 km an hour.

By comparison: Tugela Falls in South Africa, recognised since 2016 as the tallest waterfall in the world, is 983 metres high. There is no modern parallel for the Malta Escarpment waterfall, either in height, in speed or in sheer unbridled violence.

Go look again at that NatGeo relief map of the Mediterranean floor - and look east. Look at how deep it gets, and how far it extends, right up to the shores of the Middle East. (The deepest spot in the modern Mediterranean is just off Greece, where the ocean drops 5.4 km - 16,000 feet - straight down to the sea bed.)

Now try to imagine the amount of water required to fill all that - and then consider the paper’s conclusion that during this flooding event, the water-level in the eastern Mediterranean was rising by ten metres a day.

Here’s the team’s computer reconstructed summary of the proposed Zanclean Megaflood (hat-tip to F Rodriguez-Sanchez):

Nevertheless, Garcia-Castellanos notes this isn’t yet scientific consensus. In 2020, he said:

"Ten years after publishing the first observations that were related with the Zanclean flood we are still finding new evidences to sustain it, but they are not conclusive. All of the evidences that have been summarized in this article may have other possible interpretations and, before convincing the scientific community it will be necessary to have other studies that consider the hypothesis from other angles."

(However, it’s worth considering that any competing theory is going to have to work really hard to explain all these findings in other terms. There may be no universal agreement as yet, but it’s certainly looking like the most sensible explanation on the table.)

So. Whew at all that, right?

Mike Sowden @Mikeachim

I recently learned something mindblowing about the geological history of the Mediterranean Sea, and I just can't get it out of my head. Now I'm going to make it *your* problem too. Sorry. Hang onto your hat. This is wild. 1/

February 8th 2022

15,203 Retweets69,877 Likes

When I posted an abbreviated version of this story on Twitter a few weeks ago, it turned into a Megaflood of its own (a digital one).

I had geologists, oceanographers and other scientists weighing in with related research, including Garcia-Castellanos himself.

I had science fiction fans reminding me that Julian May’s Saga Of The Exiles uses the Messinian Salinity Crisis as a plot-point, by throwing the story’s protagonists back in time to the dried-up Mediterranean, and explaining the reflooding as as act of heroic terrorism.

There were mentions of the proposed engineering project known as Atlantropa - and here, I’ll just point you towards this episode (titled “The Most Ridiulous Idea Ever”) of my friend Gary’s podcast.

I had people speculating that this might be the source of all the flood myths of world history, including Atlantis, and also the one Noah escaped with his Ark. (This seems…unlikely, since modern humans wouldn’t appear for millions of years and for those stories to survive, they would have to be transmitted between the different species of our distant hominin ancestors - which were still living in Kenya at this time. Let’s just say this is the kind of stretch you won’t currently see any reputable scientist making, probably for good reason.)

There were many mentions of xkcd’s incredible “Time,” which is set during a recurrence of a Zanclean Megaflood, and might still be one of the very best things on the whole of the Internet. (Guys, thank you, and also D’OH, I can’t believe I forgot about it until you said, ugh, some geek *I* am, and so on.)

There were books aplenty. There were articles aplenty. There were science papers aplenty. I’ve never written anything that led to be bombarded with so much reading, all of it fascinating and no doubt unmissable, except, but, I’ve already got too much to read, how, how…

(How?)

All this, then, is a highly imperfect layman’s overview, written by a highly imperfect layman. Poke this newsletter hard enough and it’ll spring a leak. If I got something horribly wrong, (a) feel free to yell at me, and also (b) of course I got something wrong, I mean, have you seen me?

I’ve also failed to hat-tip a lot of people, just because so many people (hundreds!) were kind enough to fling their suggestions at me on Twitter, and I’m already dangerously close to an unreadable wordcount here.

Plus, most importantly of all, the scientific details I cite here may be contradicted at any time, because this topic is an 🚨Active Sciencing Zone🚨 where anything can happen…

But it’s already shaping up to be one of the most incredible science stories I’ve ever heard - and a perfect way to launch into this season’s topic of undersea exploration.

(Also, if this is indicative of what I’m going to find as I read my way in, I’m going to need to get a lot more coffee in.)

So - if I may, I’ll end with the same point I used to wrap up the Twitter thread, because, really, this can’t be hammered home enough:

AS YOU CAN SEE, GEOLOGY IS NOT BORING.

Thanks for reading!

- Mike

Further Reading On The Zanclean Megaflood:

- “The Zanclean megaflood of the Mediterranean: searching for independent evidence” - Daniel Garcia-Castellanos, Aaron Micallef, Ferran Estradad, Angelo Camerlenghi, Gemma Ercilla, Raúl Periáñez, José María Abril. Earth Science Reviews Vol. 201 (2020)

- “The Mediterranean nearly dried up. A cataclysmic flood revived it.” - Maya Wei-Haas, National Geographic, March 6th 2020.

- “A Megaflood-Powered Mile-High Waterfall Refilled the Mediterranean” - Katherine Kornei, Scientific American, March 26th 2018.

Thanks to:

Daniel Garcia-Castellanos, Maya Wei-Haas, Clive Finlayson, Joschua Knüppe, Dr Thomas Ronge, Harold Tobin, John Humphrey (aaaaaargh my wordcount!)….

Images: George Pentzas; NASA/Wikimedia; Daniel Garcia-Castellanos; M Library.