Here is a part of Keven Lee's book "101 Things You Never Knew About Disneyland":
A canal in New Orleans Square, labeled "1764," is all that remains of a plan to unify several themes in the land.The plan called for a crypt next to the Mansion that led into an underground catacomb of treasure and dead pirates, culminating in a pirate-themed hideout on Tom Sawyer Island. The pirate theme would have focused on Jean Laffite, a real-life pirate from the early 1800s in New Orleans. Laffite’s name might be familiar to frequent Disneyland visitors from the Pirates of the Caribbean loading zone, where a sign reads "Laffite’s Landing." The date 1764 was derived by subtracting 200 years from the birth date of one Imagineer who worked on the project. [Editor's note: this was Matt McKim, son of legendary Imagineer Sam McKim.] FURTHERMORE: Before its replacement with La Petite Patisserie, there was also a Laffite’s Silver Shop in New Orleans Square. Having a Jean Laffite identified as the "owner" of the Haunted Mansion would have united Pirates of the Caribbean with the Mansion and the island into one underlying theme, an unusual feat for an entire land. Though unrealized, the plan lives on in the form of this barricaded "crypt."
You would of entered in a crypt next to the Haunted Mansion and went down a stairway and entered the catacombs (which would be under Tom Sawyer's Island) where Laffite held his stolen treasure. You went back up to find yourself in a ship on Tom Sawyer's Island. This area would be themed to Laffite's secret hideout. It would have a lot of different things for kids to do. This idea was voted down, of coarse, but next time you go past that area with the arch, take a look at it and think about what might had been