What does Superman wear under his costume (which is worn under a striped button-down shirt)?

That's right -- he wears the Superman Black Men's V-Neck T-Shirt.

Why? Because there are occasions when saving the Earth from sentient golden bullets from Dimension G actually finishes up before 10 PM, so there's time to shed the costume, reveal the V-neck, and join the still-forming Metropolis bar crawl.

Made from 100% cotton, this Superman V-neck t-shirt for men features a bright, screen-printed, red and yellow Superman symbol.

And yes, it's an optional costume change when the closest threat is an EDM festival and NOT Braniac.