Anti-Landing Devices Found on Guernsey

From BBC--Guernsey.

They were discovered by the group Festund Guernsey on the west coast during low water on Saline Bay near Grand Rocques. They were a part of German anti-landing defense. Four were found of the six needed to build the tetrahedron-shaped object. These were never removed after German withdrawal from the island.
The British Army disposed of these devices by cutting the wires that held them together and letting them fall to the beach and slowly sink into the sand.

Gales or tidal conditions exposed them. There are still ones at Fermain, Belle Greve and Fort Doyle. Germany placed them at mid-tide point, figuring that if there was an Allied attack it would be during rising tide. At mid-tide, their vessels/vehicles would hit the mines or obstacles and sink.

--GreGen