We visited the WW2-era concrete bunker at Cape May Point. Built in 1942, it is amazing the thing is still standing, but it does have 6 ft thick concrete walls designed to take a battle ship blast.
The bunker was built to protect the coast from Germans should the United States be invaded during WW2.
After the war, the bunker was decommissioned and used as a military radio center during the 1950’s.
The local government does not allow access inside the structure, but with those thick walls, I wouldn’t expect a cell signal in that thing.