Stuck in Turtle Rock

1. I'm keeping this room for later. All it contains is a full potion of magic.
2. "2" marks the spot for a couple of chests. This room and the one next to it are made by a bunch of "bubbles" connected by a pipe system I drew in black.

Note. I escaped the left half of 1F E4 through the magic mirror, as I couldn't find any other way out.

Errata corrige: point 2 is in the middle of B1F C3, not C4 as I have mistakenly drawn.

I don't have any key. I imagine 2 holds the big key, but without a key to open that yellow passage I can't get there. I checked every room at least twice. Where did I miss out?

Unfortunately, I can't seem to recall the location of all the keys, so here's a link to a wiki with a map...Turtle Rock

The key is dropped once I killed the one of those annoying jumpy things Hokuboku in B1F, C2. This unlocks the door to the pipe bringing you to the big key.