The most valuable thing in a coal mine is not the coal.

For scientists interested in what the world looked and felt like millions of years ago, coal mines are as good as it gets. While coal may be a major culprit in global warming, there is no place like a coal mine for studying climate change in the past and its likely effects on our own world. Mining companies know this, and for whatever reason, be it good citizenship or simply good public relations, they frequently lend paleontologists a hand.

Consider, for example, Cerrejón, an immense set of open pit coal mines in northern Colombia near the Caribbean coast. The pits are huge, circular, moonscape scars in the earth with shaley slopes that dump runoff water into green crater lakes where no plant dares grow and no bird dares swim. Once in a while, dynamite collapses part of the surrounding wall, and enormous cranes collect the coal while methane fires belch from fissures in the cliffs high above.

But there’s something else. The shale slopes at Cerrejón have preserved the fossil record of an entire tropical ecosystem as it existed 58 million years ago. By looking at the fossils, paleontologists can tell what the ancient climate at Cerrejón was like (hotter and wetter than it is today) and what the foliage was like (very lush and similar to today’s Amazon jungle). The animals were huge. Cerrejón had river turtles with shells the size of kitchen tables that could seat six, and at the top of the food chain was Titanoboa cerrejonensis, a 45-foot, 2,500-pound serpent. Titanoboa was a true river monster—the largest snake ever known to have existed, and about five times the size of the Amazon anaconda, the biggest snake alive today.

What the coal shows is that Cerrejón produced these giant creatures at a time when mean ambient temperatures in the tropics were in the high 80s, about six degrees warmer than today and about as warm as temperatures can be without risking a massive die-off. Yet Titanoboa and the forest where it lived apparently thrived.

And what coal mining gives you is access to such a place. Cerrejón is the only complete ancient tropical ecosystem that is available for study. We can assume that there are plenty of fossils in the tropics, but they are buried in the jungles somewhere in the middle of nowhere. University grants can’t pay for big excavations on spec, but coal companies will, because coal is well worth the investment.

And since paleontologists and coal companies have known for at least a century that the mines are treasure chests of ancient secrets, many companies have staff scientists who look for unusual formations and alert researches when they find something good. When I traveled to Cerrejón a couple of years ago, the multinational company that runs it, Carbones del Cerrejón Ltd. did as much as possible to assist visiting paleontologists, providing everything from hard hats to radio-equipped truck drivers who knew when the dynamite was ready to go off and where to seek shelter when it happened.

Coal deposits are compressed peat swamps that began as dry-land tropical forests. Temperatures grow colder in the early phase of a natural climate cycle, and polar ice sheets expand and sea level drops. When the cycle begins to change, the ice has not yet reached its maximum extent, but rains increase in lower latitudes, changing the dry forest into swamp. The ice warms but does not melt immediately. When it does, it happens quickly and dramatically, causing sharp rises in sea level. Salt water floods the swamp, the forest dies, mud, silt and sand tamp down everything, and coal forms. Then the cycle begins again.

The good stuff—spores, seeds, pollen, cell walls and bones—is not only in the coal itself, but in the layers of sedimentary rock on the floor and the roof of each coal deposit. Think about the layers as a sandwich: The coal is the filling and the sedimentary layers are the bread. The bottom slice shows us what was in the forest when the cycle began; the top slice has everything that was there when the forest died. Each cycle, known as a “cyclothem,” lasts about 100,000 to 400,000 years, depending on where it is.

There is nothing on Earth any better than a coal mine for analyzing ancient ecosystems. The information contained in the coal is “so dense” given the compressed volume of vegetation, notes Illinois State Geological Survey geologist Scott Elrick, “that it goes with without saying that you’re going to have a huge fossil presence.”

Elrick is part of an informal network of scientists who monitor coal mining throughout the United States and the world for opportunities to study new strata as they are exposed. In 2006, Elrick and a colleague discovered four square miles of fossil forest in the ceiling of Peabody Energy’s Vermilion Grove tunnel mine in eastern Illinois, preserved elegantly 306 million years ago when a series of earthquakes allowed the sea to bury the entire forest in a single catastrophic event.