Tom Iliffe may know more about the nation's deepest underwater cave than any other explorer, but he and his associates say they still haven't reached the bottom or discovered all the mysteries that lie in the Phantom Springs cave.The Texas A&M Galveston professor of marine biology led a group of explorers in January who plunged into the West Texas underwater system for the second time. He said when searching underwater caves, a diver doesn't know how long or deep it may wind up being."You never absolutely know what's around the next corner," Iliffe said. "One day one cave will be the deepest and another day, you can find a cave that's even deeper. It's basically an ongoing challenge."Iliffe, 64, who studies the biology of underwater caves, has navigated the depths of caves in places like the Bahamas and the Yucatan Peninsula. But it was in a small West Texas town with a population of about 400 where he has now twice led teams to explore the depths of the deepest and longest underwater cave system in the United States.Iliffe dove into the Phantom Springs Cave near Balmorhea, doubling the depth explored just the previous year. On the Jan. 8 exploration, the team was able to plunge 462 feet into the cave without finding a bottom. This is the record depth for any underwater cave in the United States, according to Iliffe and an associate. In January 2012, a team explored the cave to about 230 feet in depth."There's really no telling how deep it is or how far the cave goes," Iliffe said. "Divers have been exploring this cave for more than 30 years, but there are still parts of it that no one has entered."The cave is on land owned by the U.S. Bureau of Reclamation and requires a scientific permit to explore. Few have been allowed to explore the cave because of the endangered pupfish that live in the system's external pools. Iliffe and his team are among the few who have obtained a permit.Deep-cave recordDuring the exploration, the team, which consisted of 10 experts from Texas, Florida and Tennessee, set up sediment traps, collected water-quality data, surveyed more than 8,000 feet of cave and shot hours of video and still photos.Curt Bowen, president of the ADM Foundation, the nonprofit affiliated with the trip, helped take photos and videos of the discoveries along the way."The most interesting thing was it beat the U.S. deep-cave record," Bowen said. "I didn't think it was going to go that deep when it started dropping."Besides the ADM Foundation, the project was supported by Karst Underwater Research of Florida and the U.S. Bureau of Reclamation, Texas A&M said in a news release. In 2012, A&M was able to provide partial funding for the exploration, but this year the grant expired, Iliffe said. Last month's trip was entirely self-funded, he said.Bowen, who lives in Florida, also went on the 2012 trip and said he had a feeling the cave would drop even lower. "Another trip back and we might be deeper than 500 feet," he said. "I didn't know there were cave formations in that part of Texas, especially underwater ones."Iliffe said the cave water is significantly warmer than what would normally be expected for this area, which suggests the water originates from deep below the Earth's surface. He is interested in the types of animal life present in the caves, which exist in total darkness and have adapted to live only in caves."The best way to find new animals is to go to new caves," Iliffe said.Feeds natural springThe team also discovered that the cave feeds into Balmorhea State Park, which is known as an oasis with its popular natural spring in the middle of the mountainous, desert land. They placed dye in the cave and it showed up six days later in the park's spring.Iliffe and Bowen said the team hopes to return to Phantom Springs sometime later this year to continue mapping and exploring the cave and to identify various types of cave-adapted organisms."We're going places no human has been, and we have no idea what's around the next corner," Iliffe said. "This is true exploration because we have no idea what we will find. Exploration is the only way to find out."