The Undernet, as it pertains to the Megaman Battlenetwork Series, is an area of cyberspace frequented by evil navis. It is often the most dangerous area of cyberspace, and will contain powerful viruses. It is also often the most innavigable area in the game.
Otherwise, no clue.
the door in sky or the one for undernet 2 at the end of undernet 1
From Undernet 1: go down the middle ramp, take the left/bottom path around the ramps to the arrow path behind them and take that to Undernet 2. Go right, up the ramp, and take the second warp point, then go down the ramp into Undernet 1. Wrap around to reenter Undernet 2, and from there it's a clear shot to Undernet 3. If the confuzzling layout . . . um . . . confuzzles you, a postman in Undernet 2 will give you directions.
Flamman V1 is in the Undernet, which is in found through Beach 2. Once you reach Undernet 3, plotdriven cutscenes will sheperd you along. You MUST defeat the purple navi in Undernet 3 to advance. To get to Undernet 3, follow the blue path in Undernet 1 until it widens slightly, go down the stairs and around left to the long arrow path in the back, which leads to Undernet 2. Once in Undernet 2 . . . go talk to the purple postman, who'll point you in the right direction. Flamman V2 is in a dead-end in Undernet 5. Flamman V3 is just hangin' out by the water heater in the Yoka area cause he's chill like that. Not. You'll have to find him using Oilbody.
First, you must have the Tally from one of the Tora jobs. Go to Beach 2 and talk to the purple navi guarding the warp point. This will lead to HadesNet, where you will need to equip Press in the NaviCust to proceed. Having accomplished all of that, you will enter the Undernet, home of the baddest navis, the baddest viruses, the baddest tacos, the baddest everything, man.
FlamMan V1 is in Undernet 3. He can be found with help from plot cutscenes. FlamMan V2 is in Undernet 5 in a dead end. FlamMan V3 is around the water heater in Yoka if you use oilbody. For V1, enter the Undernet. Go along the blue level until it widens a little, go down the stairs and left. Go around to the long arrow path, take it to Undernet 2. In Undernet 2 go right, up the stairs to the blue level, and take the second warp point. Then down the stairs into Undernet 1. Wrap around into Undernet 2, take the path to Undernet 3. The plot will guide you from there. YOU MUST FIGHT THE PURPLE NAVI IN UNDERNET 3 TO ADVANCE. If you keep saying no, you'll never get anywhere.
Take the warp point to the top left of Undernet 2, follow the path around to the purple navi guarding the way. Plot permitting, he'll let you pass, and that is Undernet 4.
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FlamMan V1 is in Undernet 3. FlamMan V2 is in a dead-end in Undernet 5, and FlamMan V3 is in random battles in Yoka 1 by the water heater with Oilbody equipped in the NaviCust. To reach the Undernet, go to Beach 2 and talk to the purple navi guarding the warp point. If you have the "Tally," and you should at that point in the game, you will be allowed through. The warp will take you to HadesNet, and once across the press-path you will enter the Undernet. From Undernet 1, go along the upper level until it widens, then go down the ramp and head left/down, and go around the ramp to the long arrow-path. Take that to Undernet 2. Immediately go right and up the ramp to the upper level. Take the second warp point and go down the stairs back into Undernet 1. Wrap around and reenter Undernet 2. Take the path to Undernet 3. From there cutscenes will guide you to FlamMan. You MUST fight the purple navi in Undernet 3 to advance.
You get to Netfrica and go west. Then you should be in Sharo. Use your C-Slider to slide though the line and you should be in the Undernet.
In Megaman Battle Network 6, It doesn't have The Undernet 3, but it has Undernet Zero where you can fight Bass at there.
Defeat FlamMan's V2, available in a dead-end in Undernet 5. Go to Undernet 6, and go up and left as far as possible. Through trial and error you will eventually get to an isolated path leading up and left, until you hit FlamMan's security cube. It will dissolve, and you will be free to continue on, to the top of the area and thence to Undernet 7.
There's only one: through Undernet 2, via Undernet 1. That's it. The other option is coming from the UnderSquare, but I don't think that's an option for you. For instructions on how to use the original route, ask another question.