Both the drywell and the wetwell are surrounded by a secondary containment vessel (or shield building) also built from reinforced concrete about a meter thick.
The drywell is a large steel pressure vessel that looks like a giant upside-down pear and holds the reactor and primary pumps, and the wetwell is a large toroidal vessel that looks like a donut.
Normally, circulation would channel the hydrogen and oxygen to a recombiner where they would be restored back to water, but in the hours after the reactors were shut down, hydrogen was accumulating and separating in the wetwell and reached a point where it was vented into the sparse steel-frame structure at the top of the reactor building.