It contained the skulls, many of them smashed in, of previous prisoners who had been held there. The First Doctor, Ian Chesterton, Barbara Wright and Susan Foreman were taken captive by the tribe and imprisoned there, before they were to be sacrificed to Orb. They initially were able to escape through a secret passage at the back of the cave, shown to them by one of the tribe.
After they were recaptured, they again escaped, this time by creating a distraction through use of torches and the damaged skulls. Pursued by the tribe, they were able to reach the safety of the TARDIS. (TV: An Unearthly Child)