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.
https://kotaku.com/look-out-hes-got-a-piece-half-life-2-mods-mario-5406072
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.
https://kotaku.com/left-4-dead-meet-the-mushroom-kingdom-5380354
Left 4 Dead + Super Mario Bros. [Botchweed]
