If you are looking for Jailbreak Tunnel, go here.
"Jail is a place where bad people go."
Jail is a building in PMU 7 located northeast of Grassroot Town, found by following the route out of town north and then far to the east, then north again (past Sour Root Cave). Near it is Jailbreak Tunnel, a dungeon claimed to be the path escaped convicts use to get out of the jail.
Rule breakers can get sent here for punishment sometimes, and are unable to get out unless Staff release them.
Inside the Jail, the place is gloomy and a little dark. There is a table where players can socialize, a broken column, and the jail gates. Then there's Aggron the Jail Warden, who keeps the jail secure and has a Sneasel and Murkrow locked up tight in the cell to the left. Judging by the skull inside the cell, he does his job very well.