The underbelly_130909_2521.zip file you have requested: Maps Levels Missions / Coop Maps / Left 4 Dead Coop Map Underbelly - 2.31mb is not available. Perhaps try browsing all files for the game Left 4 Dead and upload any files you want to share or are missing. This is the only information we could locate:
Relive the scenario from Resident Evil Outbreak File#2 with a L4D twist. Also try this map in versus mode. Next update will have textures and fix issue of infected getting stuck in vents.