On December 27, 1966 the Cave of Swallows, the largest known cave shaft in the world was discovered in San Luis Potosi, Mexico. (Of course it was already known for centuries to the native Huastec people.) The floor of the cave is a 370 meter (1,214 ft) freefall. New York City’s Chrysler Building could easily fit inside it. The cave is popular with extreme sport tourism. Rappelling to the floor takes about 20 minutes. It is also popular with BASE parachuting (buildings, antennas, spans, earth.) Several sites have related that an average-sized hot air balloon was navigated through the 160-foot opening and landed on the floor, but I was never able to discover who did this or when.

This image, A flock of conures in the Cave of Swallows, La Huasteca, Mexico, was photographed by Onixmtz and is made available with a GNU Free Documentation License.