Undernet

The Undernet, known as Ura Internet (ウラインターネット) in Japan, is a thematic end-of-game area from the MegaMan Battle Network series. It is a hangout and home for criminal Navis that consider most NetNavis from the surface to be weak and spineless. Rarely, denizens of the Undernet will interact with the overworld Cyber Network, usually to deliver a message, or to carry out an order. The Undernet is dark, and generally characterized by faded colors and static that symbolizes bad reception for an electronic device.

The Undernet was originally a creation of SciLab to develop and preserve the powerful program known as Giga Freeze. It was created as a vast area away from the regular Net so if an accident were to occur, the damage would be contained. Due to it's remote nature and lack of constant interfacing, it became popular as a hiding place, and then a home for criminal NetNavis. It is considered taboo by many regular NetNavis. It is also the home of Bass.EXE and Serenade.EXE, who frequent different parts of the abyssal area. The Undernet has several deeper areas connected to it. In MegaMan Battle Network 3, it connects to the eerily peaceful Secret Area. In MegaMan Battle Network 4, it connects to a world of darkness called Murkland, otherwise known as the "Black Earth". In MegaMan Battle Network 5, a Mr. Prog opens a portal to Nebula Area in the Undernet. In MegaMan Battle Network 6, it connects to a shadowed land known simply as the Graveyard. The Graveyard, as the name states, is the realm where all remaining data of deleted Navis are stored in graves.

The Program of Light and Dark
The Undernet is rarely seen and mentioned in the anime. Unlike the game version, the undernet resembles a void rather than an evil area. Forte describes it as an "electronic graveyard", as data remnants can be seen floating around.

The Undernet also contains the Wily Program, which contains a beast known as Nebula Grey. It is revealed that Dr. Regal discovered the program after he and LaserMan were thrown in after being destroyed by R-Rockman.