Some of the magic rocks are evil.  

Well now he is irrationally sympathetic to the bloody rocks like they have feelings. Why is he like this. 

Nod nod. Then once any dangerous rocks have been retrieved for destruction, back.... into the water. Joy. Are they sure Gulnan didn't have a secret ground entrance or anything? 

He could try navigating the corpse-warrens in a general upward direction, if he likes.

Hmm. Well. That... also sounds somewhat horrible. Does Jojo have a preference. 

"I'm willing to try navigating if we can maintain a map, so that we can return to the water exit if our search is fruitless."

"Oh, a map! Great plan." He will prepare to attempt to draw a map on the back of his erstwhile department meeting notes. He's not a cartographer but he has the steady drafting hand of a person who's ever been personally responsible for the seaworthiness of a ship. 

The catacombs do admit of mapping! And they're not, upon investigation, totally labyrinthine; there's only a few major branches.

Some of them do, of course, contain undead.

They should stop containing undead, then. 

This is relatively straightforward to accomplish and, moreover, something of a public service, which makes Jojo noticeably happier.

They come to a door which leads them into a basement, rather than more catacombs! This seems like progress. Upstairs from the basement there is chanting.

Excellent. He is getting a good grade in Professor (subtype: support and encourage the children in accomplishing goals suited to their skillset and which they find soul-exalting). 

... more chanting. Joy.

Well, on the one hand, last time the strategy of charging in to prevent whatever evil nonsense the chanters are Up To went really very terribly. But on the other hand they are now pretty specifically better prepared for it. Sneak up the stairs and attempt to interrupt whatever that is?