Tunneling

It's not man made. This tunnel is actually created by molten lava. Wouldn't say it would you? It's called a lava tube. Tubes form in one of two ways: by the crusting over of lava channels and from Pahoehoe flows where the lava is moving under the surface. Pahoehoe is a sort of basaltic lava. This Thurston Lava Tube in Hawaii Volcanoes National Park.