Marion Smith, a relentless, irascible subterranean explorer who was believed to have visited more caves than anyone else in human history, died on Nov. 30 at his home in Rock Island, Tenn. He was 80.

The cause was congestive heart failure and chronic lymphocytic leukemia, said his partner, Sharon Jones.

His fellow cavers called Mr. Smith “the Goat,” and he certainly looked the part, with a compact, wiry body and a wispy caprine beard dangling below a well-cragged face.

He was likewise goatish in his implacable determination to keep going through mud and cold and scraped shins, with little patience for those who couldn’t keep up. He was still caving long past the age when most people would decide to hang up their head lamps: His personal record for most cave visits in a year — 335 — came in 2013, when he turned 71.

But mostly he was the Goat because he was roundly considered the Greatest of All Time. He explored 8,291 separate caves — far more than anyone on record, ever. He climbed up and down some two million feet of rope.

He was especially taken with vertical caving: He descended more than 3,000 underground pits deeper than 30 feet, often dangling freely in the abyss on a rope no thicker than a thumb.

“If caving were a professional sport, Smith would possess the lifetime stats of a Wilt Chamberlain or Ted Williams,” Michael Ray Taylor wrote in a 2003 profile of Mr. Smith in Sports Illustrated.

Humans have been going into caves since the origin of the species, but it was only in the 1960s that cave exploration took off as an organized activity in the United States. The Europeans came earlier, in the 1930s and then again after World War II, with high-profile expeditions by Spanish and French explorers into the vast caverns of the Pyrenees.

Some of those cavers wrote books, and their accounts, translated and published in America, helped set off a wave of interest among college students and other young people. Caving became especially popular in the South, and in particular the triangular region where Tennessee, Alabama and Georgia intersect, beneath which lies a vast deposit of cave-friendly limestone.

It was also home to Mr. Smith, who grew up outside Atlanta and studied history at West Georgia College (now the University of West Georgia).

He entered his first cave in 1966 and was immediately hooked. With the advent of space travel, the last unknown parts of the earth’s surface would soon be revealed. But the earth’s interior was still a mystery, and every cave was full of surprises: exotic creatures, underground rivers, tiny passages that opened suddenly to cathedral-like spaces.

“Every day, he wanted to create an adventure,” Chuck Mangelsdorf, a lawyer and fellow caver, said in a phone interview.

Mr. Smith developed a reputation as the guy who seemed to be everywhere, every weekend, constantly announcing new finds, pushing into unknown spaces without a whiff of fear. In 2014 he was pinned under a boulder for nine hours. Three years later he was hit in the temple by a fist-size rock that fell from 40 feet. In both cases he went to the hospital, and in both cases he was back underground within days.

Like most serious cavers, he was offended by the term “spelunker,” which to them denotes unprepared, unserious subterranean venturing. He enjoyed the tedium of cataloging and mapping every inch of a cave as much as he reveled in pushing through to the next unknown.

In 1998 Mr. Smith was part of a team of cavers who discovered a 4.5-acre, 350-foot-tall underground chamber in East Tennessee they named the Rumble Room. They kept it secret for four years while they explored and mapped it, and they revealed it to the public only when a nearby town threatened to use an adjacent cave as part of a new sewage system.

“I didn’t want to let the cat out,” Mr. Smith told The Tennessean newspaper in 2002. “I wanted to keep it in the bag longer.”

Marion Otis Smith was born on Sept. 24, 1942, in Fairburn, Ga., the only child of Otis Smith, a farmer, and Bernice (Stephens) Smith, a homemaker. His parents later divorced, and he was mostly raised by his grandparents.

After receiving bachelor's and master’s degrees in history from West Georgia College, he entered the Army and served two years in South Korea. He was discharged in 1969.

Back in Georgia, he spent several years working different jobs, the sort that paid little but asked little in return, allowing him to spend as much time as possible underground. Eventually he cleaned up, a bit, and in 1974 he was hired as an assistant editor at the University of Tennessee, charged with preparing the 16 volumes of President Andrew Johnson’s papers for publication. He retired in 2000.

With Ms. Jones, he finally bought his first home in the early 2000s, on a backcountry trail north of Chattanooga called Bone Cave Road. He was married once, but just briefly. Ms. Jones is his only immediate survivor.

Caves were his life, but exploring them was not his only passion. He was perhaps the world’s leading expert on the history of mining for saltpeter, a primary ingredient in gunpowder, which in the 19th century was often harvested from caves.

In the 2010s he joined with Joseph Douglas, a historian at Volunteer State Community College in Gallatin, Tenn., in a project to document the thousands of signatures left by Confederate and Union soldiers in Mammoth Cave, in central Kentucky. Mr. Smith was particularly taken with researching the men themselves, and he ultimately wrote about 80 miniature biographies.

“He called it the history of the obscure, but it took a great level of patience and attention to fine detail,” Dr. Douglas said in a phone interview.

But mostly, Mr. Smith kept pressing deeper into caves he knew, and scouring the woods around East Tennessee for the hidden entrances to new ones.

“Even if I’m physically impossible to go in a wild cave, surely I can be put in a wheelchair and wheeled to a commercial cave,” he told The Chattanooga Times-Free Press in 2014. “And if I can’t be sitting up in a cave, surely they can put me on a stretcher and wheel me into one.”