I've never heard of this. I tried a quick google search and this is all I came up with, but it's really fascinating that this was considered and begun! It's from Wiki Fandom:
Unused
In the 1990s, a proposed mega-theme for New Orleans Square would have had
catacombs going underneath of the Rivers of America in-order to connect to Tom Sawyer's Island. This would have been accessed via the crypt of
Jean Lafitte within the cemetery of the Haunted Mansion. Remnants of this unused installation can still be found throughout Tom Sawyer's Island and New Orleans Square.