According to police, crews were dispatched at about 4:09 p.m. to the beach entrance near Seaview Road on the barrier island.
Police said 18-year-old Levi Caverly, of Maine, and his 17-year-old sister were digging a large hole when it 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 neighboring towns also responded to the scene to assist in rescue efforts, police said.