The "Gadget" device detonated at the Trinity site involved two explosions: first a conventional TNT explosion and then a nuclear explosion, if the chain reaction started by the first explosion was maintained. The Jumbo device was designed by the X2-A section of the Los Alamos laboratory to act as a failsafe device for the Trinity test explosion. General Groves spent $12 million on Jumbo, a steel cylinder 10 feet in diameter and 25 feet long. Its walls were 14 inches thick and the entire device weighed 200 tons.
Jumbo was designed to contain the Gadget and prevent the loss of the precious plutonium in the case that the conventional explosion succeeded, but the nuclear explosion failed. It took a combination of rail and a specially constructed 64-wheel trailer to deliver Jumbo to Trinity. However, by the time of the actual Trinity test, Manhattan Project officials were confident enough in the plutonium bomb and secure enough in the stream of plutonium coming from Hanford, that Jumbo was deemed unnecessary. Jumbo was suspended from a tower during the test, but it survived the nuclear explosion.
General Groves was concerned that Congress would criticize him for spending $12 million on what was essentially a white elephant, so he ordered Jumbo destroyed. However, eight 500-pound bombs only succeeded in blowing the ends of it. The remains of Jumbo can still be seen at the Trinity site.