In 1797, in central Kentucky, a hunter chased a bear into a hole in a cliff … and discovered the longest cave network in the world, 10 million years in the making.

But at that time, all he could see—and smell—was bat guano, which was then the valuable source of saltpeter, a key component in gunpowder.

Within a year, prospectors were mining centuries of bat droppings for military efforts. During the War of 1812, the gunpowder business boomed, and so did the cave’s mine. In 1815 the war ended, and so did the mine.

The next year, Mammoth Cave, named for its enormous size, opened as a tourist attraction. The slaves who had been the miners now served as tour guides.

They showed the cave to Gilded Age patrons, who carried oil lamps and used their smoke to write their names on the ceiling.

In the 1840s, a freed slave named Stephen Bishop, an expert cave guide, was the first to explore and map the cave.

After a century of tourism, it was thought to comprise more than 40 miles of caverns and connected passages.

But new scientific exploration and technology, continuing to the present day, has revealed it’s actually 10 times that size. And more rooms are expected to be found.

Today, the cave’s petroglyphs and speleothem formations are seen by half a million visitors a year. If you find yourself in Kentucky, be one of them!