At approximately four hours into their descent–several thousand feet above the sea floor–a sharp clang sounded through the pressure sphere and the vehicle shuddered violently. Once their wincing subsided, the men did what they could to inspect the craft and its condition. It seemed that the water pressure at this never-before-encountered depth–six tons per square inch–had cracked the outer pane of the lucite window. For the moment the vehicle itself remained watertight, but the damage was worrisome. The Trieste was outfitted with a few safety systems; for instance, the ballast doors were held closed by electromagnets, so in the event of electrical failure the doors would fall open and drop the ballast, causing the vehicle to rise to the surface. But such systems would be of no help to the men inside if the 1,000 atmospheres of pressure crushed their delicate passenger compartment. Moreover, no other vehicle in existence was capable of reaching such depths, which meant that if her float tank became compromised there was no chance of rescue. Nevertheless, the stalwart scientists opted to press on.
It was also the last time anyone dived that deep. Like the space race, once it had been done, no one saw the use in continuing to pay for such risky adventures. Read the entire story at Damn Interesting. Link