Why is iwo jima considered one of the bloodiest battles that occurred on the pacific front?

The American amphibious invasion of Iwo Jima during World War II stemmed from the need for a base near the Japanese coast. Following elaborate preparatory air and naval bombardment, three U.S. marine divisions landed on the island in February 1945. Iwo Jima was defended by roughly 23,000 Japanese army and navy troops, who fought from an elaborate network of caves, dugouts, tunnels and underground installations. Despite the difficulty of the conditions, the marines wiped out the defending forces after a month of fighting, and the battle earned a place in American lore with the publication of a photograph showing the U.S. flag being raised in victory.

Where is the map? it's not showing