DIO was most likely recovering and staying hidden in the coffin, using it as a safe place from sunlight after the events of Part 1. The fandom’s common read is that he was healing, conserving strength, and waiting for an opportunity to reemerge, rather than doing anything mundane or secretive in a literal sense.

What the story implies

In JoJo’s Bizarre Adventure , the coffin functions as a survival container: it keeps DIO out of the sun and lets him endure until he can act again. Some fans also speculate he was in a kind of suspended recovery state after the body-related changes and injuries he had gone through.

Why people argue about it

There isn’t a single explicit scene that spells out every minute of those 100 years, so the gap gets filled with fan theories. Common guesses include:

  • He was sleeping.
  • He was healing.
  • He was simply waiting in darkness until someone found him.

The most likely answer

The simplest answer is: he was lying low in the coffin, surviving and recovering. The coffin is less about “doing something” and more about DIO avoiding sunlight and preserving himself until the next stage of his plan.

Note

There’s also a separate coffin-related context in Vampire Survivors , but that’s about unlocking a character and has nothing to do with DIO’s canon role in JoJo.