Tim Willits level The Wind Tunnels is one of the greatest in the game, and is the only level other than Termination Central in the third episode that isn't themed in the dark metal style. It consists of a series of enormous underground rusted metal rooms in some sort of water filtration facility, connected by a series of air tubes (the tunnels of the title) that can be used to propel yourself from area to area.

The level doesn't have much variety in its monsters, not many are suited to the environment here. You'll face mostly Scrags, Ogres and Fiends, along with the occasional Death Knight, and a Shambler later on in the level. Some of the environmental features here cause trouble for the monsters, the Fiends often get caught underwater, for example, but the gameplay is excellent nonetheless.

E3M5 is an excellent deathmatch and team deathmatch level as well as single-player, but doesn't see quite enough rotation on the servers considering this. The tunnels used in the level appear seldom elsewhere in Quake, either in custom levels or the official expansions. One of the levels in the second Quake Mission Pack, Dissolution of Eternity's Towers of Wrath, has a number of them, but they are not used to the great effect found here, and otherwise are few and far between.

The pre-release and beta versions of The Wind Tunnels were completely empty, not a monster, item or secret in sight (or out of it for that matter) and the tunnels featured as a secret level rather than one following the standard course of the episode.

There are hundreds of other differences between these betas and the final cut; the runes are known as sigils and the epilogues presented on completion of each episode use sample copy (stating “That's the end of episode <number>! When you have the sigils from all the episodes, you can face Shub Niggurath!”), Chthon and the Vores have different skins, and both throw fireballs instead of the standard Vore magical attacks and a number of other levels are missing small sections or contain differences in architecture and style.

The most significant changes are in the game's grim finale, Shub Niggurath's Pit, and the episode two level The Dismal Oubliette. The Pit was a completely different level, still the work of Romero but Shub herself is the only thing common between the two and his Oubliette still contained it's ‘Lost Entrance’.

You can read more about the various alternate versions of Quake and it's sequels, their console ports and their design and development here.