Gameplay
See also: Half-Life (video game) § Gameplay

Comparison of the Anomalous Materials lobby room in Half-Life (top) and Black Mesa (bottom)
Black Mesa is a first-person shooter that requires the player to perform combat tasks and solve various puzzles to advance through the game. From a design standpoint, the core gameplay remains largely unchanged from the original base Half-Life game; the player can carry a number of weapons that they find through the course of the game, though they must also locate and monitor ammunition for most weapons. The player's character is protected by a hazard suit that monitors the player's health and can be charged as a shield, absorbing a limited amount of damage. Health and battery packs can be found scattered through the game, as well as stations that can recharge either health or suit charge.

However, unlike Half-Life: Source, which merely featured the original game's assets and geometry ported to the Source engine, Black Mesa has been purpose-built from the ground up to take full advantage of the newest versions of Source, not just for its graphical capabilities, but for its myriad updates to the game's physics engine, puzzle complexity, and platforming capability. The artificial intelligence of the enemy characters has also been improved over Half-Life to provide more of a challenge, with some of the combat spaces redefined to provide more options to the player.[1][2] In addition, several narrative and design changes have been made to account for the numerous story threads presented via retcon in Half-Life 2. While most of the general design and progress through the game levels remains the same as Half-Life, the largest change in Black Mesa is the reworking of the game's final chapter, Xen, which was generally considered the weakest part of the original game.[1]

Black Mesa also includes support for the individual and team deathmatch multiplayer modes from Half-Life on similarly-updated maps.[2]

Plot
See also: Half-Life (video game) § Plot
The plot of Black Mesa is almost identical to Half-Life's storyline.[3] Like in the original game, the player controls Gordon Freeman, a theoretical physicist working at the Black Mesa Research Facility. He is tasked to place a sample of anomalous material into an Anti-Mass Spectrometer for analysis, using the Mark IV Hazardous Environment Suit (HEV Suit) to do so safely. However, the sample causes a "resonance cascade", devastating the facility and creating an interdimensional rift to an alien dimension called Xen, bringing its alien creatures to Earth. Freeman survives the incident, finds other survivors, and is tasked to make his way to the surface to call for help. Upon reaching the surface, however, he finds that the facility is being cleansed of any living thing - human or alien - by the military. Freeman learns from the surviving scientists the only way to stop the alien invasion is to cross over to Xen and destroy the entity keeping the rift open.

Development
See also: List of video games derived from mods
Initial efforts (2005–2012)

The "Surface Tension" chapter as it appears in Half-Life

The same scene in a development version of Black Mesa
With the release of Half-Life 2 in 2004, Valve re-released several of its previous titles, ported to their new Source game engine, including the critically acclaimed 1998 game Half-Life as Half-Life: Source. The Source engine is graphically more advanced than the GoldSrc engine used for the original games. Half-Life: Source features the Havok physics engine and improved effects for water and lighting. The level architecture, textures, and models of the game, however, remained unchanged. Half-Life: Source was met with mixed reviews. IGN liked the new user interface and other technical features, but noted that it did not receive as many improvements as Valve's other Source engine ports.[4] GameSpy said that while it was a "fun little bonus", it was "certainly not the major graphical upgrade some people thought it might be".[5] Valve's managing director Gabe Newell is quoted as saying that a complete Source remake of Half-Life by its fans was "not only possible…but inevitable".[6]

Black Mesa began as the combination of two independent volunteer projects, each aiming to completely recreate Half-Life using Source. The Leakfree modification was announced in September 2004. The Half-Life: Source Overhaul Project was announced one month later.[7] After realizing their similar goals, project leaders for both teams decided to combine their efforts; they formed a new 13-person team under the name Black Mesa: Source.[6] The "Source" in the project's title was later dropped when Valve asked the team to remove it in order to "stem confusion over whether or not [it was] an endorsed or official product", which at the time it was not.[8] Eventually, the team rebranded itself as the Crowbar Collective. Most of the team was distributed across the world and used online collaboration to work remotely, with some limited in-person meetings.[7]

Originally based on the version of Source released with Counter-Strike: Source in 2004, the project switched to a more recent version released with Valve's The Orange Box in 2007. This new version included more advanced particle effects, hardware-accelerated facial animation, and support for multi-core processor rendering, amongst other improvements.[9][10][11] The team had expected this to be a relatively fast project, with trailers released in 2005 and 2008, and an initial release estimate of late 2009, but by mid-2009, had backed off that date, and changed their expected release date to "when it's done".[12] Wired included the game on their "Vaporware of the Year" lists in 2009 and 2010.[13][14] In the lead-up to the 2012 release, team member Carlos Montero said that in 2009 that they thought they were going to be able to make that date, but "ended up busting our asses to make that a reality, and we went against a lot of our core values in the process. We found ourselves rushing things, cutting things, making quality sacrifices we did not want to make."[15] Montero said then they decided to re-evaluate the state of the project, set higher bars for the quality of work they wanted to produce, and started to back through what they had already done to improve upon that, at which point they were not sure when the project would be completed.[15]

The first standalone version of Black Mesa was released as a free download on September 14, 2012.[16][17] This contained remakes of all Half-Life's chapters except the final chapter set on the alien world Xen, which the team intended to rework for inclusion in a future release, as Xen in the original Half-Life was often considered its weakest part.[18] The development team estimated that the initial release of Black Mesa gave players eight to ten hours of content to complete.[18] Black Mesa's initial release coincided with the launch of Valve's Steam Greenlight program which allowed users to vote for games to be put onto the Steam storefront. Black Mesa became one of the first ten titles to be voted on by fans and approved by Valve to be included on Steam through Greenlight.[19]