This small cavern on the southeast side of the Skull Pool has deep gashes in the walls where it looks as though many people used to set mooring anchors for small watercraft.
The south side of the cavern is collapsed, and beyond it was once the enclave of an enigmatic group of magi known as The Keepers, who managed and maintained the lock. The cavern beyond was collapsed by Xanathar, who became paranoid about the Keepers being treacherous after a Xanathar Guild vessel came out of the locks too rapidly and was dashed against a cave wall.
It is possible to crawl through the rubble to access the cavern, but it is difficult and risky.