How do you get to black earth 1 in Megaman battle network 4

First, you need a completed six souls. Then go to the UNDERNET 4, there's a door that requires the union of six soul. Enter that door and the go straight.
You are in the UNDERNET 5 now. You must also complete the standard chips to be able to defeat BASS. after you defeated bass, there will apear 5 heel navi. Defeated them to enter the black earth.