With the outbreak of the Second World War, Mare Island rapidly added some bomb shelters for the workers and residents.  These concrete bunkers were built quickly but have lasted a long time.  The doors to them have gradually decayed and they are now predominantly sealed with steel doors.  As bomb shelters go, they were of reasonable use.  A direct hit would have destroyed them and whoever was unlucky enough to be inside.  However, they should have provided protection to the occupants from shrapnel resulting from a nearby impact had it ever happened.  Now they just look a little incongruous amongst the trees but there would have been a time when they were considered a very welcome feature to those nearby.