The Sunken Ship Theory
Could it be that a pirate ship was purposely sunk and then buried on Oak Island? This theory was explored in Season Three of the show, when Fred Nolan showed Rick evidence of a ship in a swamp on the island. Nolan also believed that the swamp was artificial.
Nolan explained that the swamp was created to hide a ship buried down deep. The team then went to the location with a professional diver and discovered a plank of wood which they assumed was part of a ship.