Peleliu already contained excavations from phosphate mining, and associated wharves helped bring in equipment for its defenses. Engineers fitted caves with sliding armored steel doors that concealed and protected artillery, machine guns, and troops. The doors could open for firing and close for shelter. Slanted cave entrances diverted grenade and flamethrower attacks away from the interior. A tunnel and trench system through central Peleliu let defenders evacuate and recover positions along internal routes. Troops could continue moving with substantial protection while artillery struck the surface.
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Peleliu's cave engineering protected guns and internal movement
Former phosphate workings, armored doors, angled cave entrances, and internal routes let Peleliu's defenders shelter and move under bombardment.