WHITE SANDS MISSILE RANGE, N.M. (AP) — Military officials in southern New Mexico are preparing to host visitors at the spot of the world's first atomic test during a special one-day open house at the Trinity Site. The first of two annual open houses will be April 7. At the site on White Sands Missile Range, visitors can take a quarter-mile walk to ground zero where a small obelisk marks the spot of detonation. Historical photos are mounted on the fence surrounding the area. Visitors can also tour the Schmidt/McDonald Ranch House, where scientists assembled the bomb's plutonium core. Last July marked the 70th year anniversary of the test at the Trinity Site. It was part of the Manhattan Project, a top-secret World War II nuclear development program out of the then-secret city of Los Alamos.