Loose change and other items often found under or between furniture cushions (especially couches), automobile seats, and other such places where stuff falls out of your pockets and you usually can't find it without crawling on your hands and knees.
Origin: This is a common enough phenomenon that I decided there needed to be a term for it. It is a play on words from the phrase "undersea treasure".
College dude 1: “Man, I need a Mountain Dew for tonight’s cram session, but I spent my last quarters on the laundry machines…”
College dude 2: “Look around the dorm lounge, dude. There’s bound to be at least 50 cents in underseat treasure there…”
College dude 1: “Thanks, bro; I never thought of that!”
College dude 2: “That’s ‘cause you’re a pothead. Anyway, try to steer clear of the used condoms.”
Prices shown in USD.