Police said crews were dispatched around 4.09pm to the beach entrance near Seaview Road on the barrier island.
Police say Levy Caverley, 18, from Maine, and her 17-year-old sister were digging a large hole when he collapsed and trapped them.
The 17-year-old was rescued and treated at the scene, police said.
Caverley did not survive the collapse, police said. Crews were still working to recover his body.
Emergency crews from nearby towns also responded to the scene to help with rescue efforts, police said.
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