Important to keep in mind: 1200 years ago, the Atlantic Ocean would have been at least 12 feet/3.65 meters lower than today, and Mahone Bay shorelines would have been significantly further out, perhaps as much as 50-feet/15.25-meters, to about 100-feet/30.5-meters, or more.
An obvious example of the erosion and rising sea, is to be found at Westhaver's Island, which is number 4 across from the Mader's Cove Clay-Shaft. In the 1750's, when the area was plotted for properties that could be acquired by immigrants, Westerhaver's Island was large enough to have had a small farm, as well as a lighthouse and a house for the light-keeper's family. Now, that island is about the size of a triple garage.
Oak Island Unearthed - exploring the Mayan / Aztec connection (Good article, check it out)
If the water level was much lower, it would be possible underground caverns were dry and used by people around 500-1000 AD.
Money pit could have been a ventilation shaft.
One theory is the Aztec, riding the Golf stream, had been mining for gold & clay and may have later been used to hide Montezuma's body and treasure when Mexico was invaded.
So instead of Seven must Die, It could be "Montezuma's Revenge."
Your Bud Aurum