From Board Game Online Wiki

Superman is an event that can be reached when The United Nations launch an Atom Bomb while a player has the The Iron Giant skill.

In the event, the Atom Bomb is destroyed, the The Iron Giant skill is lost, and a Giant Iron Screw lands in a random(?) player's inventory.
The following flavor text appears in Superman event:
[PlayerName]'s Iron Giant looks at the sky, realizing he is the only one able to stop the incoming atom bomb. He kneels down and gently speaks to [PlayerName]:
"[PlayerName]... You, stay. I, go. No, following."
With an amazing blast he flies off into the sky towards the incoming nuclear missile. His final thoughts are that he is who he chooses to be...
A blinding explosion fills the sky and it is clear that not even our very own Iron Giant could have survived such a destructive force...
One Giant Iron Screw tumbles down from the sky and happens to land in [PlayerName]'s inventory. Maybe there still is some hope for the future left.