The invasion of Tarawa Atoll was the second American offensive in the Pacific after Guadalcanal. On 20 November 1943, an aerial bombardment of carrier-based Dauntless dive bombers preceded the amphibious landings. The shore was defended by a maze of bunkers and interlocking gun positions so well-entrenched that the preliminary bombardment had little effect. Once the bombing lifted, the island's defenders were able to take up defensive positions and began repelling incoming landing craft. The majority of the Higgins boats got hung up on the barrier reef surrounding the island, forcing the Marines inside to jump over the side into neck-high water and wade hundreds of yards to shore, all the while under constant machine gun and artillery fire. Amphibious tractors, also known as Amtrak's or LVTs (Landing Vehicle Tracked) fared slightly better as they were designed to climb over obstacles and land on shore. Over the course of the day, more than half of the Allied landing craft were destroyed by Japanese mortars and light artillery.