From early 1944 until the days leading up to the invasion, Kuribayashi transformed the island into a massive network of bunkers, hidden guns, and 11 mi (18 km) of underground tunnels. The heavy American naval and air bombardment did little but drive the Japanese further underground, making their positions impervious to enemy fire. Their pillboxes and bunkers were all connected so that if one was knocked out, it could be reoccupied again. The network of bunkers and pillboxes greatly favored the defender.
Two advantages of the underground tunnel and bunker system were that the bunkers could be reoccupied after being knocked out and what else?
impervious to enemy fire