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://lastchance.cc/look-out-hes-got-a-piece-half-life-2-mods-mario-5406072%3C/a%3E%3C/p%3E
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://lastchance.cc/left-4-dead-meet-the-mushroom-kingdom-5380354%3C/a%3E%3C/p%3E
Left 4 Dead + Super Mario Bros. [Botchweed]