||The door was built a long time ago by thieves who were trying to hide their stolen treasures. I don't think an ordinary key is going to open that door.
The Thief's Door (盗賊のトビラ, Touzoku no Tobira) is an event in Tomba!.
Unlock the door in the Underground Maze.
Activate the event by talking to the digger inside the small house in the Underground Maze that can be found by going down the hidden elevator in Watch Tower.
In order to unlock the door, Tomba must obtain the Thief's Wire from the 10,000 Year Old Man by clearing the The 10 Math Beads event.
Go down to the door in the Underground Maze and select the item from the Item Screen to unlock the door. The event is cleared.