User blog comment:Diego Brando/Phantom Blood (Doubt about coffin)/@comment-5919632-20131203033250

For me, it looks like the coffin had a fake bottom, with another one hidden behind the one normally seen.

This is the explanation given in the Jorge Joestar novel, where Dio is hidden in the true bottom. But that novel, as bizarre as it is, makes Erina actually find out about Dio in there (With Jonathan's body) and the two make a deal: In exchange for a little of Erina's blood, Dio will help her survive in the ocean (cooking fishs with Space Ripper Sting Eyes, etc). Not exactly sure, but its something along these lines.

However, this would mean that Dio arrived in the coffin BEFORE Erina, something hard to believe as she left the battle place first.