The smell.
When a nuclear submarine is at sea, it spends most of its time submerged. The atmosphere inside the submarine is sealed. Part of the function of the submarine's atmospheric control system is to remove carbon dioxide (CO2), as its buildup can be fatal. To remove carbon dioxide, the submarine uses a chemical called amine . When the amine is cold, it absorbs carbon dioxide, and when it is hot, it releases it. The amine is then recycled through a machine called a CO2 Scrubber, which alternately heats and cools the carbon dioxide and pushes the gas overboard, allowing the crew to breathe.
This system is very effective, but the downside is that the amines impart a rather "unique" odor to the atmosphere. This eventually permeates every part of the submarine's interior, including the crew members' clothing and even their skin.
In addition to the amine odor, the submarine crew was exposed to the smells of cooking, hydraulic oil vapor, diesel exhaust, sanitation tank ventilation, and the odors of numerous sanitation tanks, as well as the odors of people living together in the stuffy space. The interior of the submarine became extremely smelly. The crew members became accustomed to it and, after a while, never noticed it. But others did.
While I was serving on a submarine, I had an excuse to go home and take a vacation in my civilian clothes. I was sitting in my airplane seat next to an older woman, making small talk, and suddenly she asked me, "Did you work on a submarine?" Surprised, I asked, "Yes, how did you know?" She replied, "My husband used to be on a submarine. I'll never forget the smell."