Update: The leaked map in question, though indeed from a canceled Half-Life 2 episode, was not a formal "Episode Three" but a spinoff that never saw the light of day. It's possible that it would have been the third Half-Life 2 episode released, but was not planned as "Episode Three" by name.
Original Story: Recently, a relatively large data dump from an an unknown source was turned over to the world, its content mainly concerning "the original map files for the Half-Life 2 episodes." While source files from early-to-mid-2000s PC games may not exactly be the most fascinating under normal circumstances, that changes when the files are from games that were never released. In combing over the files, Valvetime.net has concluded exactly that.
The site's analysis determines that one of the map files in the leak, a snow-themed version of a familiar Half-Life locale, is in fact from the unreleased third Half-Life 2 story episode. Valvetime's analysis is as follows.
It is implied that the player crashes into a warehouse in a gondola. He wakes up in a room with two unique characters named Duncan and Scooter. There is a train station and buildings nearby. A group of rebels and Combine Soldiers fight on the streets.
Perhaps the most shocking angle here for some players will be the simple fact that late 2005, the stretch during which the canceled episode was being developed, was almost 12 years ago. You can check out further analysis of the leaks here, though don't say I didn't warn you when it comes to drudging up old wounds about the Half-Life franchise and, of course, the now tirelessly memed theoretically third game.
Valve may not be pouring its efforts into Half-Life, but it is keeping busy - you can read our four-part interview with the legendary developer, publisher, and Steam-creator to brush up on what's new.