The Atomic Cauldron is a disintegration pit located beneath the Fortress of Solitude, built by Superman to disintegrate safely any dangerous artifact or substance he stumbled on.
Fueled by radioactive elements, the pit can dissolve almost anything - including a Kryptonian - into their molecular components.
On one occasion, Superman and Supergirl dumped several highly-destructive weapons in the Cauldron. Unfortunately the fumes given off by burning bio-chemical weapons drove them temporarily crazy. As they fought, Supergirl fell into the pit. The sight of his cousin about to die made Superman snap out of his madness. Quickly he pulled Supergirl out of the Cauldron and both flew out of the Fortress where they recovered from the mind-warping gas.
Several years later, Kara Zor-El was tossed in the Cauldron by several copies of herself. Fortunately Supergirl managed to escape, clinging to the walls of the pit and flying out before plunging into the radioactive soup.
Links and References