Rescue crew started to search for four university students exploring a cave in Texas.
Three women and a man who attend the University of Texas went into the narrow passage, called Airman's Cave, on Saturday morning and told friends to call for help if they were not back by midnight, said Austin Fire Department Lt. Matt Cox.
The cave is more than 12,000 feet (3,657 meters) long and is difficult for even experienced cave explorers, Cox said. The "keyhole" entrance is less than 18 inches (45 centimeters) across.
"It's really, really long, very small and very tight," Cox said. "They could be lost. They could simply be fatigued or tired."
Rescuers have found water bottles and cell phones apparently left behind by the four students, Cox said. By Sunday afternoon, 14 rescuers were helping in the search and were waiting to hear from a team that had reached the end of the cave, he said.
There are few places in the cave for the students to get hurt because it is so narrow. Rescuers have left food, water and medical supplies throughout the cave.
"They've been in there so long that they probably don't have any light," Cox said. "Their batteries are probably dead."