Bottomless Pit - Carlsbad Caverns National Park, New Mexico - Deep Hole Inside a Huge Hole

Tours through Carlsbad Caverns typically start with the Big Room, a massive underground chamber than includes the world's largest stalagmite, hundred-foot high ceilings, and stalactites that resemble chandeliers. One of the most popular features of the Big Room is a deep rift in the limestone floor that seems to extend downward without limit - the so-called "Bottomless Pit.''

Originally, the pit was really thought to be bottomless. When stones were tossed into it, no sound was ever heard of them striking the bottom. Later explorations revealed the truth. The bottom was deep, about 140 feet below the opening, and covered with loose soil. When the stones reached the bottom, the sound of their landing was muffled by the soft dirt. Still, visitors love to put the pit to the test. As a result, park rangers have to rappel to the bottom once a year to retrieve all the trash that's been thrown in.

Another interesting aspect of the Bottomless Pit is the large amount of gypsum around it. The soft white mineral attests to how the chamber and pit were formed. When sulfuric acid combines with limestone and water, it releases carbon dioxide and water, leaving gypsum deposits as a byproduct. Over the course of 250 million years, rainwater seeping down through cracks in the ground and sulfurous gases rising up from the Earth's depths gradually dissolved the soft limestone to create these amazing formations.