The Underground is a location from the Mega Man Battle Network series. It is a giant hole in Central Area and is where the Cybeasts were sealed when they rampaged on the Net. There is a Bug Frag Trader Machine located there. The Underground is a two-part area with a giant platform in the second. Whenever a character enters, there is a large roar.
After MegaMan.EXE is taken over by the Cybeast in Mega Man Battle Network 6, he retreats to the Underground but is followed by the Link Navi that Lan Hikari chooses. However, the Cybeast's presence has caused evil spirits to flood the Underground. A Netpolice unit is sent down but defeated. The Link Navi uses the Soul Weapons to defeat the Evil Spirits and makes his way to the giant platform in Underground 2. There, the Link Navi does battle with Beast Out MegaMan and barely wins. CircusMan.EXE arrives and attempts to steal MegaMan but fails. Then Colonel.EXE arrives and fights MegaMan but retreats after losing.
Later, Bass.EXE comes and gains the power of the Cybeasts by absorbing fragments of their data. He revives a copy of Beast Out MegaMan but MegaMan defeats it. Then Bass attacks MegaMan himself but loses. It is the last story-triggered battle against Bass (he can be randomly found in the Graveyard after defeating him here). Also, after the battle, BeastMegaman in his SP form can be found in this area.