The best part of hiking Diamond Head is that you have to be in Hawaii to do it. This easy hike of about 500 vertical feet is a quick and enjoyable event to do while visiting Oahu. The trail goes through a military installation established in 1906 and added on into WWII. According to the Hawaiian Encyclopedia and http://www.diamondheadhike.org/militaryhistory.html “A network of tunnels were carved into the mountain, and cannon emplacements were placed atop the crater rim along with observation posts and bunkers. Fort Ruger was reinforced during World War II, though the guns were never fired.” The trail takes you through these tunnels and bunkers. At one point you have to crouch through a bunker opening to continue on the trail. Very cool. The trail then loops down and around the summit back to the trail so you can avoid entering the bunker if you choose.