Complex sentence: The actual formation of the Cave of the Mounds is similar to many caves, a large crack in the surface of the rock, called the cave's "lifeline", allows rain water to seep into the stone.
Split into simpler sentences: The actual formation of the Cave of the Mounds is similar to many caves. A large crack in the surface of the rock, called the cave's "lifeline", allows rain water to seep into the stone.