This image shows manhole to periscope wall, valve wheels for flooding and blowing. Hanwheels for periscope gear, air pressure gauges. The UB-110 sunk after attacking a merchant shipping convoy near Hartlepool in July 1918. It was then salvaged and transferred to Swan Hunter Wigham Richardson Ltd. Dry Docks (Wallsend), with an order to restore her to fighting state. The order cancelled following Armistice and she was scrapped thereafter.
Imagine having to be in there during combat, being on a UBoat was the most dangerous place in the war. Also not many good ways to die in a sub. Drowning, instant decompression if you sink in deep water or if you sink in shallow waters you’d probably sit on the seabed until the oxygen is gone. During WW1 Germany had 351 operational boats, sunk in combat: 178 (50%), other losses: 39 (11%), completed after Armistice: 45, surrendered to Allies: 179, men lost in Uboats: 5,000 killed.
From a fictional U-Boat story (Neal Stephenson, Cryptonomicon):
It seems like a strange way to kill people. Shaftoe’s not sure if he approves of everything that is implied by this U-boat. Shaftoe has killed Chinese bandits on the banks of the Yangtze by stabbing them in the chest with a bayonet. He thinks he killed one, once, just by hitting him pretty hard in the head. On Guadalcanal he killed Nips by shooting at them with several different kinds of arms, by rolling rocks down on them, by constructing large bonfires at the entrances to caves where they were holed up, by sneaking up on them in the jungle and cutting their throats, by firing mortars into their positions, even by picking one up and throwing him off a cliff into the pounding surf. Of course he has known for a long time that this face-to-face style of killing the bad guys is kind of old-fashioned, but it’s not like he’s spent a lot of time thinking about it. The demonstration of the Vickers machine gun that he witnessed in Italy did sort of get him thinking, and now here he is, inside one of the most famous killing machines in the whole war, and what does he see? He sees valves. Or rather the cast-iron wheels that are used for opening and closing valves. Entire bulkheads are covered with iron wheels, ranging from a couple of inches to over a foot in diameter, packed in as densely as barnacles on a rock, in what looks like a completely random and irregular fashion. They are painted either red or black, and they are polished to a gleam from the friction of men’s hands. And where it’s not valves it’s switches, huge Frankenstein-movie ones. There is one big rotary switch, half green and half red, that’s a good two feet in diameter. And it’s not like this boat has a lot of windows in it. It’s got no windows at all. Just a periscope that can only be used by one guy at a time. And so for these guys, the war comes down to being sealed up in an airtight drum full of shit and turning valve-wheels and throwing switches on command, and from time to time maybe some officer comes back and tells them that they just killed a bunch of guys.