Back on I'd Rather Be Surfing (see map). So that's where that locked door lead.
Monsters to kill:
Skitters and maybe the odd Ghast if you go looking for them
Decision to make:
You see a vine ladder going up. Ascending to Lasciate Ogne Speranza, Voi Ch'Intrate.
You see a vine ladder going down. Descending to Warning: Earthquake Zone.