Underround

A one-winner event whose outcomes collectively cost less than 100 cents to buy. Since exactly one outcome must pay 100 cents, an underround field is priced below fair value, the prediction-market version of an arbitrage.

Underrounds are rare and short-lived: they usually appear when one outcome's price goes stale or the book thins out, and they vanish as soon as someone notices. The catches are real, fees per contract can erase a thin underround, filling every leg at size moves prices, and a stale quote on one thin outcome can fake the whole thing, so every leg needs verifying before acting.