What DHARMA station was hidden in the hatch?

The DHARMA station hidden in the hatch was The Swan. It was an underground research facility concealed on the island and discovered by John Locke and Boone Carlyle after Oceanic Flight 815 crashed.

The Swan was DHARMA’s third known station. Its original purpose was electromagnetic research, but after a dangerous incident, it became a containment facility. Workers were instructed to enter the numbers 4, 8, 15, 16, 23, and 42 into a computer every 108 minutes.

The Swan is often confused with The Pearl, another underground DHARMA station. The Pearl monitored other stations and treated the Swan’s occupants as subjects in a psychological experiment. The Flame, by contrast, was a communications station, while The Orchid focused on time and space research.

The hatch became one of Lost’s central locations because it connected the survivors’ daily struggle with the island’s older scientific mysteries. Its destruction released a massive electromagnetic burst and briefly made the island detectable from outside.