Later exploration revealed the bottom was about deep and covered with soft dirt. The stones made no sound when they struck the bottom because they were lodged in the soft soil. On October 31, 2013, a cave technician exploring the Spirit World area discovered a new chamber hundreds of feet up from the main area. Dubbed "Halloween Hall" for the date of its discovery, the fresh find marks the biggest discovery for the caverns in more than 25 years. The room's diameter is about , and more than 1,000 bat bones were discovered inside the room. The park contains over 119 caves. Three caves are open to public tours. Carlsbad Caverns is the most famous and is fully developed with electric lights, paved trails, and elevators. Slaughter Canyon Cave and Spider Cave are undeveloped, except for designated paths for the guided "adventure" caving tours. Lechuguilla Cave is well known for its delicate speleothems and pristine underground environment. Guano mining occurred in the pit below the entrance in the 1910s. After gaining permission from the national park managers to dig into a rubble pile where wind whistled between the rocks when the weather changed, cavers broke through into a room in 1986. Over of cave passage has been explored and mapped. It has been mapped to a depth of , making it the second deepest limestone cave in the U.S. To protect the fragile environment, access is limited to permitted scientific expeditions only. Seventeen species of bats live in the park, including many Mexican free-tailed bats. It has been estimated that the population of Mexican free-tailed bats once numbered in the millions but has declined drastically in modern times. The cause of this decline is unknown but the pesticide DDT is often listed as a primary cause. A study published in 2009 by a team from Boston University questions whether millions of bats ever existed in the caverns. Many techniques have been used to estimate the bat population in the cave. The most recent and most successful of these attempts involved the use of thermal imaging camera to track and count the bats. A count from 2005 estimated a peak of 793,000. The Mexican free-tailed bats are present from April or May to late October or early November. They emerge in a dense group, corkscrewing upwards and counterclockwise, usually starting around sunset and lasting about three hours. (Jim White decided to investigate the caverns when he saw the bats from a distance and at first thought they were a volcano or a whirlwind.) Every early evening from Memorial Day weekend to mid October (with possible exceptions for bad weather), a ranger gives a talk on the bats while visitors sitting in the amphitheater wait to watch the bats emerge.