Now that it is no longer working on Evolve, developer Turtle Rock Studios apparently was scrounging around in its old files and came across something that Left 4 Dead fans should find interesting ... and playable: An unreleased campaign.

Studio co-founder Chris Ashton said on the game's official forums that the add-on Dam It was never finished. "It's gray box, and some code features are missing but it's playable from beginning to end," he said. "It also includes two standalone survival maps that never shipped." The campaign was originally supposed to fit in between Dead Air and Blood Harvest, with players starting on a crashed C130 that they used to escape in Dead Air. Turtle Rock actually streamed this add-on a few months ago on their official Twitch channel.

Players who want to try it can download it and install it, but Ashton pointed out a few of the missing code elements:

• Fire, smoke and explosions do not trigger panic events in the hangars.
• Zombies do not crawl all the time in the orchard.
• Farm house mob does not spawn in the far side of the woods.
• Covered bridge does not collapse with explosions.
• Burned forest area is missing wandering fires.

Also of note:

• The burning forest section has a burn trigger that stays on too long. If you move quickly you'll take burn damage.
• The dam spawn room is missing health kits and weapons.
• The elevator models have an offset origin in the dam. Players can still get on and use them, but they are invisible and wall collision is missing.

Here is a video of the level and what you can expect: