Or specifically, the barrel of a Japanese 200mm cannon on Peleliu. The World War II cannon was dug into the rocks and virtually unreachable with sniper holes guarding the entrance. The problem? As it was so deeply dug into the rocks it had a very limited field of view and effectiveness.
The Americans snuck up the side to take it out.
The artillery and mortar caves of the Japanese were well thought out. Many had steel doors on them, which would close during US naval or artillery bombardment – popping open the moment the shelling stopped. This was a big gun.
It was very well dug in. Too well in fact.
The sniper holes are under the moss.