The Alpha Door

1. The mysterious vessel is included for the sake of historical accuracy. "Jumbo," as it was affectionately known, was built to contain the Trinity device and prevent the loss of its precious plutonium in the event of a misfire. In the end, the scientists decided not to use Jumbo, and hung it near the shot tower to see what would happen to it. (The steel framework was utterly demolished, but Jumbo survived more or less intact.)
2. Don't worry. You don't need Jumbo to complete the story.