The Hot Zone

From the text:

"The cave is large enough to hold seventy elephants at a time."

The mouth of the cave is huge - fifty-five yards wide - and it opens out even wider beyond the entrance. They crossed a platform covered with powdery dry elephant dung, their feet kicking up puffs of dust as they advanced. The light grew dim, and the floor of the cave rose upward in a series of shelves coated with green slime. The slime was bat guano, digested vegetable matter that had been excreted by a colony of fruit bats on the ceiling.

Bats whirred out of holes and flicked through their flashlight beams, dodging around their heads, making high-pitched cries. Their flashlights disturbed the bats, and more bats woke up. Hundreds of bat eyes, like red jewels, looked down on them from the ceiling of the cave. Waves of bat sound rippled across the ceiling and echoed back and forth, a dry, squeaky sound, like many small doors being opened on dry hinges. Then they saw the most wonderful thing about Kitum Cave. The cave is petrified rain forest. Mineralized logs stuck out of the walls and ceiling. They were trunks of rain-forest trees turned to stone-teaks, podo trees, evergreens. An eruption of Mount Elgon about seven million years ago had buried the rain forest in ash, and the logs had been transformed into opal and chert. The logs were surrounded by crystals, white needles of minerals that had grown out of the rock. The crystals were as sharp as hypodermic syringes, and they glittered in the beams of the flashlights.