The Rock Maximum Security Prison

With crime becoming a real problem, something had to be done about the most serious of reoffenders and prison escape artists. So, on a remote rocky island of unknown coordinates in the middle of a desolate, turbulent, shark-infested ocean, a maximum security prison was built to house the worst criminal masterminds, and occasionally adventurers causing mayo-based shenanigans. Designed to be utterly impossible to escape from, The Rock is manned by an elite team of guards. If inmates do manage to escape the fortress, they'd have to test their chances in the inhospitable ocean around it. Only one person ever made it this far, and they rowed back, preferring jail than certain death.