Like a Dragon: Infinite Wealth

Both the North and South Yokohama Underground entrances can be found in north west Ijincho – you'll find it behind Cafe Brave, near Kinka Street. To enter, talk to the Geomijul Agent standing outside. The Yokohama Underground has three floors to tackle, each with their own sequence of sectors and missing people to rescue. There are fifteen captives to rescue in all, so be sure you're the right level to face the dungeon, gear up and get ready to grind. The final floor requires level 44 and up, but you'll be duly rewarded upon completing it. Once you reach each floor's midpoint, you can also exit and then return later to pick up where you left off. Make sure to recover health and save whenever you're able to (at the designated spots in the dungeon). Beat all three floors to unlock this.