The Deepdown is a subterranean region beneath Sturmhalten. It's first mentioned (and entered) , as Kalikoff is receiving a lesson from Veilchen as regards what qualifies as "scary." Subsequent events, however, suggest that either Veilchen overestimates his own capacity for inducing fear, or underestimates the Deepdown's.
According to Kalikoff's fellow plumber Sturvin, the Deepdown has a long-standing reputation for being prime monster habitat, and it's worsened lately. Some 25 years ago or so, trips into the Deepdown were -- after a fashion -- with rewards and a grand party thrown by the Sturmvoraus prince when monsters encountered there (usually resulting from something the prince had cooked up, of course) were dispatched. Nowadays, however, the denizens of the Deepdown have assumed such a level of nastiness that "it ain't no fun at all!" to go down there, as Kalikoff discovers the hard way. (Remarkably, he survives.)
These historic facts all come to light as Veilchen and the plumbers are guiding the rescue party (Zeetha, Krosp, Lars and Da Boyz) through the sewers of Sturmhalten in search of a route into Castle Sturmvoraus so they can rescue Agatha from the clutches of the Sturmvoraus family. He directs the merry crew into the Deepdown under the pretense of looking for secret passages into the castle, the ones in the "normal," upper levels of the sewers allegedly not getting to where everyone wants to go. One large, many-eyed, glowing squid later, it's clear that this wasn't such a great idea, and the party flees in perfectly justified terror -- but into the Deepdown, not out of it. This is not good. Having gotten them into this mess, Veilchen rather inconsiderately , evoking a certain professional admiration from Maxim even though he and the other rescuers have been left to die.
Well, they don't die, of course. Kalikoff reappears to save the day, and all manner of additional merry adventures ensue, as the rescuers first Geisterdamen habitat (although they're evacuating it, for unknown reasons), then that it's also of green glowy things that even the Geisterladies want nothing to do with. These are evil enough to compel a hasty retreat, Agatha or no Agatha; the party (minus part of Dimo, but nothing a Heterodyne can't fix) eventually re-emerges topside just in time for the real fun.that the Deepdown is prime
We may not have seen the last of the Deepdown, even though both the GGTs and the Geisters have decamped, the former to the surface to join in the fun at the Battle of Sturmhalten, the latter to who-knows-where. The Good Guys will probably have to deal with the Geisters somewhere, somehow, and their only known trail starts in the Deepdown, so more fun and games may remain to be enjoyed here.