^ But there was no pressure release nor did David show signs of any resistance when opening it. This suggests one of two things, either Spock had an in-built release switch (which would be stupid, since he was dead) which broke the airtight seal, or the Casket wasn't sealed at all.
If it wasn't air-sealed, then it's quite frankly impossible that Spock's corpse would have managed to make the trip to Genesis (and creating a massive plot hole in the process), given the effects of the vacuum of space on organic matter (the force would surely pull Spock's corpse from inside the coffin, if not crush the casket into a ball!), not to mention what happens with an atmospheric entry. The casket would have been made out of duranium, presumably, but even that would be useless unless it was properly sealed - as all space ships are.