Haunted Mansion catacombs.
One of the original ideas in New Orleans Square was to pull every ride (Pirates, Tom Sawyer, Haunted Mansion) under a unifying theme.
This attraction was a walk through catacomb that led from outside the Mansion to Tom Sawyers island, to a new underground area called “Pirates Lair.”
The idea was there was a bloodthirsty pirate who owned the Mansion and was alluded to in the POC ride. The bodies in the catacombs were the 999 Happy Haunts. Why they were happy is a mystery.