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Black Earth (ブラックアース Burakku Āsu) is a secret area in Mega Man Battle Network 4, where the player can fight DS Navis and later on MegaMan DS and Bass. It can be accessed from UnderNet 5 under certain conditions.
Getting to Black Earth is a major task. First, MegaMan must have all six Double Souls, obtained through beating the game twice prior, to access Undernet 5. Next, all the Standard Chips are required to open the door that leads to the end of that area. After that, Bass must be fought and defeated. The final task is gathering the five great Evil Chips to break the seal on Black Earth: BugCharg/BugCurse (depending on the version), Anubis, BlakWing, Muramasa and ElemDark. The bug Giga Chip is obtained from the battle Mystery Data won in battles with ShadeMan SP or LaserMan SP. The ElemDark is found from ElemperorEX in Undernet 6. The other three are held by HeelNavis that appear after Bass is defeated, and must be won through 10 battles against viruses each.
Black Earth 1
Black Earth 2
A circuitous area, this place contains powerful goodies such as a HPMemory, 3000 Zenny, a GigFldr 1 Navi Customizer program and the BugFrag Trader. As MegaMan moves inward, he must fight Dark Soul Navis of the opponents he has Double Souls of, earning a DS Navi chip from each one. Once the center is reached, a warp hole takes MegaMan to an item called the Mirror of Truth. This leads to a boss battle with MegaMan DS. Winning earns the SoulClen Navi Customizer Program. The light next to the mirror will also give the HubBatc NaviCust Program.
If MegaMan returns to this area after beating MegaMan DS, Bass SP appears as a random encounter, dropping the BlakAura/DeltaRay Giga Chip if beaten.