Appears to be a scorched earth area perhaps a flame thrower or flame throwing Sherman silenced the bunker as they were very stoutly built pretty much impervious to most stuff.
They found a flamethrower burned them up & used up all the oxygen in the bunker causing the demise of the occupants if not by flame then lack of 02 pretty brutal way to go but the Pacific was a brutal affair with an unyielding foe.
RIP to all the allied service personnel that went through those islands always on patrol.