The Trinity Site near Socorro, New Mexico is where the US tested the first atomic bomb on July 16, 1945. The site is open to the public twice a year, on the first Saturday of April and the first Saturday of October, as it's now on the White Sands Missile Range. It's a fascinating visit, with the obelisk (added later) located exactly at ground zero. The site is still mildly radioactive, although you'll get more radiation from flying in an airplane! Pieces of sand that were melted and fused by the heat of the atomic blast (which measured about 20,000 tons of TNT) are called "Trinitite", a kind of green glassy material that's located in some places on the ground. It is forbidden to take Trinitite from the site. 
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