At about Kilometer 107 on Highway 186 there is a small turnoff on the north side of the road, just big enough to pull your car off the road. From there, a couple of hundred meters of walking through the trees and up a small hill brings you to a huge sinkhole, at the bottom of which is a narrow cave opening.
At dusk every night, hundreds of thousands of small black bats emerge from the cave in an endless whirling rush. Known locally as la nube negra (the black cloud), it is an incredible spectacle and unlike no other.
At the height, you can feel the wind from their collective wings and winding upward flight—a few may even crash into you or stop for a rest on your pant leg. (No worries—they’re completely harmless to humans, feeding mostly on insects).
Although you may be able to find it on your own, it’s better to stop in the morning at Río Bec Dreams (Carr. Escárcega-Chetumal Km. 142, tel. 983/124-0501, www.riobecdreams.com ) in Xpujil  and arrange for a guide (US$10–15).