Hydrogen pods were storage units used by the Gorn Hegemony in the starship construction yards on the planet of Seudath. As the name suggests, they hold hydrogen. In the year 2268, Starfleet Intelligence agent Bridget McLellan was attempting to stop the Klingon Imperial Intelligence operative Goloth. During the chase she fired her phaser at Goloth and hit a hydrogen pod instead. The resulting explosion started a series of explosions that lead to the destruction of the half-completed starship they were fighting on. (VAN - Declassified novella: The Stars Look Down)