21 July 1961. Mercury-Redstone 4 (nicknamed Liberty Bell 7 by the pilot because of the bell-shape), launches into space. It is the United States' second manned space flight. Inside was astronaut Virgil Grissom on his first ever flight. Before launch, there was found to be a misaligned bolt on the hatch, but engineers said that the 69 bolts that remained were enough, and the bolt wasn't replaced.
The flight went according to plan, until Grissom came in for the splashdown. The hatch blew too early, and water began to flood into the capsule. Grissom was rescued just in time. The helicopter that was supposed to carry it to solid ground struggled to hold onto the capsule, as the water pouring in was making it too heavy.
Eventually, the tether snapped from the extra weight, and Liberty Bell 7 fell into the water, sinking to the bottom.
Although an investigation attempted to blame Grissom for the loss of the capsule, he went on to further flights. On 27 January 1967, Grissom was killed when a fire broke out in the Apollo 1 capsule, ironically because of the faulty hatch.
In 1999, a crew went out to seach for the Liberty Bell 7 capsule and attempt to raise it from the ocean. The following is the documentary of their journey.