Eeurgh, that high-up switch in the apparent dead-end. Despite reading the spoiler guide instructions over and over, I had to check a Rampancy.net video to figure out where the damn thing was supposed to be.
The Juggernaut encounter down in the heart of the volcano wasn't much fun, either; it's a bit difficult to find a decent spot to pelt it from, and IIRC I got blown up a few times (once, possibly because of improper SPNKR usage).
Can't even remember how many tries Mt. Vesuvius 2 took, but it was quite satisfying to finally escape.
And man...you'd think that Pinky and the Brain wouldn't mesh well with Marathon, but they managed to make it semi-plausible.