Desmond exposed to the electromagnetism.
They intended to use it to test whether or not Desmond Hume could survive another electromagnetic phenomenon. They tested it before putting Desmond inside. However a circuit failed and Simmons was sent to inspect it. Another worker found the fault, fixed it, and immediately switched the power back on, before Simmons was given time to exit. Simmons was killed by the electromagnetism and Desmond was forcefully put inside next. Desmond survived the event and was left unconscious. Although only unconscious for a few seconds, he experienced a portion of his life in the flash-sideways timeline. ("Happily Ever After")
The chamber was located in a yard outside one of the Hydra station buildings. It was an enclosed white wooden structure, containing two large solenoid coils, with huge cables connecting it to the main building. When switched on, the solenoids would generate an intense electromagnetic field. A large metal stairway led up and into the main building from there.