Lucrecia's Cave

The Crystal Caves is where Lucrecia Crescent stayed after the birth of her child, Sephiroth. During the events of Final Fantasy VII: Advent Children and Dirge of Cerberus -Final Fantasy VII-, she is encased in the glowing crystals and is able to talk to Vincent Valentine telepathically. This place holds a lot of importance to Vincent and the story line of Dirge of Cerberus.

Final Fantasy VII
In the original game, the Crystal Cave could only be reached by using a submarine and going through an underwater channel. By emerging at the end, the entrance to the cave can be seen. Inside is Vincent's long lost love, Lucrecia Crescent. You can view a series of flash-backs that show the entire tragic tale of Hojo and Sephiroth's conception. When you leave the cave and fight ten random battles, Vincent's final Limit Break and ultimate weapon, Chaos and Death Penalty can be found here when you return.

Dirge of Cerberus -Final Fantasy VII-
The Crystal Cave and Lucrecia plays an important role in -Dirge of Cerberus- Final Fantasy VII. After the death of Sephiroth, Vincent would visit Lucrecia and talk to her telepathically every now and then. It is later revealed that Lucrecia encased herself in the Crystal because she felt guilty for the "death" and transformation of Vincent, and also for bearing the sin of having given birth to Sephiroth. In the epilogue, Vincent would speak to Lucrecia one last time, thanking her for giving him a new lease of life. A tear can be seen rolling down Lucrecia's cheek as Vincent leaves the cave.