Half-Life: Uplink was the second official demo for Half-Life, released on Febuary 12th, 1999. Unlike the Half-Life: Day One demo, Uplink was distributed freely over the internet. It contains 3 short levels not seen in Half-Life. They follow an alternate timeline Gordon Freeman as he fixes a transmitter dome and sends a signal to the USNRC, to open a set of doors at the beginning of the first level, supposedly leading to safety. The demo ends with Gordon being trapped behind rubble as a Gargantua walks toward him. The screen fades to the black with the message: "Current evaluation: uplink completed. Require further data."
Febuary 12th, 1999
Chat log with John Guthrie explaining some of the development of Uplink. Source.
"We had two goals for Uplink. The first was to introduce new players to the essence of the Half-Life experience.
The second was to give current Half-Life players a brand new experience. Creating a mini-mission that wasn't part
of the core Half-Life story served both purposes. It was a blast to go back and create a "missing" mission, with
all the energy and intensity of the full game, but sad to say there aren't any "unknown" missions lying around
waiting to be discovered...with the possible exception of any that designers in the custom map community might
be putting together!"
- Marc Laidlaw
Source: Half-Life 2: Raising The Bar (Uncorrected Proof) (Page 158).
"By the time we finished Half-Life, we wanted to streamline the development process. The Uplink mission pack tested
that out. One person built the world, another did all the entity work to create a whole experience; a handful of
people who put it together with audio that we had left over. There were a bunch of scenes in Half-Life that we
wanted to do and weren't able to pull off. In some cases we already had audio. So Uplink was a separate mission
about tuning the antennae to the satellite you launched when it was up there. From there we thought, 'Let's build
the next game this way.'"
- Marc Laidlaw
Source: PC Gamer Interview.