I know that submariners are supposed to be made of stern stuff but today’s invention is a low-tech way to help improve their living conditions.
Interior designers aren’t supposed to be made of stern stuff, but they do know about how to make small spaces seem much bigger. One way is by using mirrors.
Today’s invention is to fit mirrors (plastic, impact-safe ones would be fine) to the inside surfaces of some bulkheads and cabinets on board submarines. Although the Captains Nemo wouldn’t necessarily want to view their stubble close-up, the occasional reflective patch would provide much better light distribution and an increased sense of space for people in cramped conditions.
(I imagine a windowless Mars-bound spaceship would benefit similarly).