* The Raft: This is a common Marvel prison, frequently seen in various comics and media. It's likely the most recognizable prison for supervillains in the Marvel universe. It's possible it was mentioned in the series, though it's not a central plot point.
* The Cube: This is a highly secure prison featured in the "Earth's Mightiest Heroes" series. It was the primary holding facility for supervillains, and it featured prominently in the episode "The Breakout."
Additional Possibilities, Though Not Featured in the Show:
* 42nd Street Maximum Security Prison: A prison that serves as a holding place for villains in the Marvel Cinematic Universe. It would be plausible for this facility to exist in the "Earth's Mightiest Heroes" universe.
* The Vault: This prison is specifically for powerful and dangerous villains in the Marvel Comics. It's possible the show's creators might have drawn inspiration from it.
It's worth noting that the show might have used other unnamed prison facilities as well. However, the focus wasn't on the specifics of supervillain prisons. The show instead focused on the heroes' battles with villains.