From the longer Wikipedia page  which has a list of theories.
Oak Island is a 140-acre (57 ha) island in Lunenburg County on the south shore of Nova Scotia, Canada. The tree-covered island is one of about 360 small islands in Mahone Bay and rises to a maximum of 35 feet (11 m) above sea level. Located 200 metres from shore and connected to the mainland by a modern causeway, the island is privately owned.
Oak Island is noted as the location of the so-called Money Pit and the site of over 200 years of treasure hunting: there is a 'strange inscription' associated with the Pit.
For over two centuries treasure hunters, archeologists, engineers, and researchers have been trying to solve the mystery of what might be buried or concealed on Oak Island. Publications, books, documentaries, and programs have featured the island, and it was the subject of a television series on the History Channel, The Curse of Oak Island. The television show takes its name from the local legend that Canadian folklorist Helen Creighton included in her book Folklore of Lunenburg County, Nova Scotia about how seven people must die before the treasure will be found. Theories of what could be buried range from pirate gold, Spanish treasure, Knights Templar treasure, a Rosicrucian vault, King Solomon's temple treasure, religious artifacts such as the Ark of the Covenant, ancient Egyptian tomb, or the lost Shakespeare manuscripts.
There are many 19th-century accounts of Oak Island, but some are conflicting or biased. Further, physical evidence from the initial excavations is absent or has been lost. There have been a number of attempts to dig it subsequenty with no success. A range of theories as to what (if anything) has been buried has been proposed.
There has been wide-ranging speculation among enthusiasts as to how the pit was formed and what it might contain.
Another article is at