Half-Life's introductory tram ride through the Black Mesa Research Facility is legendary. It's also where you can catch your first sight of the mysterious G-Man, the suited observer that quietly follows you through the game. The eruption of the Anti-Mass Spectrometer causes a Resonance Cascade; a tear in the fabric of space that allows a legion of alien creatures to pour into Black Mesa.

The US Marines arrive at Black Mesa, but their aim isn't to save the staff. Instead they'll kill every witness and cover up the incident. Half-Life's first major boss is the Tentacles in 'Blast Pit'; a blind, triple-headed creature that must be tricked through sound.

The fight with the Gargantua in 'Power Up' showcased the most dangerous alien yet; a colossal armoured beast with flamethrower hands. In 'Surface Tension' you finally make it out of Black Mesa, but the surface is just as dangerous. Thankfully laser-guided rockets are on hand to bring down the Marines' helicopters.

The mystery of Black Mesa becomes ever murkier in 'Questionable Ethics', in which you discover that your colleagues knew about the aliens all along and have been secretly experimenting on them. The true extent of Black Mesa's secrets are revealed in 'Lambda Core'; the lab's elite science team has been visiting the alien's home world using an advanced teleporter. And - surprise! - you get to be the next one to use it.

Half-Life completely changes gears when you are transported to the trippy alien world of Xen. The alien .