This looks a lot like the re-creation of World 1-1 in Half-Life 2 - it and Left 4 Dead use the same engine and toolkit, after all. But Left 4 Dead Super Mario Bros does include subterranean rooms.
Bill's jumping is of little use in this board though - everything's built too high. He busts out blocks the old fashioned way, with firepower. For the pipes and the final ascent, the creator, Chubdzoomer, evidently put in ladders.
Still a work in progress, says Chubzdoomer, with enemies and an exit to come later. For a more fully-featured Super Mario experience with zombies, try DoritoClock's Super Mario Bros. 64 mod in Left 4 Dead.
Left 4 Dead + Super Mario Bros. [Botchweed]