Underground is built upon the skateboarding formula of previous Tony Hawk's games: the player explores levels and completes goals while performing tricks.

Underground was developed with a theme of individuality which was manifested in the extensive character customization options, the presence of a narrative, and the product's characterization as an adventure game.

Underground was a major critical and commercial success, with reviewers praising its wide appeal, soundtrack, customization, multiplayer, and storyline.

[4] Underground introduces the ability to dismount one's skateboard, which allows the player to explore levels more carefully and reach new areas.

[6] On account of the levels' large sizes and the integration of goals into the story, Underground has been described as an adventure game.

[2] Underground features many real world professional skateboarders including: Tony Hawk, Bob Burnquist, Paul Rodriguez, Chad Muska, Mike Vallely, Andrew Reynolds, and Stacy Peralta.

After impressing Tony Hawk, the protagonist wins the Best Trick event at Tampa AM and is offered deals by major skateboard sponsors, much to Eric's dismay.

The protagonist then heads to San Diego, California to meet Todd, the manager of the team, and completes several photo shoots for a magazine.

Finding a tall hotel, the protagonist climbs to the roof and recruits Eric to film a trick video atop it.

After designing their own pro skateboard, the protagonist and Eric embark on a team trip to Moscow, Russia, where they reconcile.

If the story has been completed more than once on two different difficulties, an alternate ending occurs, where the protagonist knocks Eric unconscious, taking the tape back instead of holding the skate-off.

[7] One reason for only allowing the player to use a custom character was that certain criminal acts completed in the plot would not reflect well on real-world skaters.

[14] Most of the levels were modelled closely after real-world locations; the designers traveled to locales representative of each city and took photographs and videos as reference.

[15] Development of the basic gameplay mechanics and structure began quickly but by the end of August 2003, only two months before the American release, work was still in progress.

[18] Activision promoted Underground with the "Tony Hawk's Face Off Mobile Tour", a series of events across 29 cities in October.

[20] The console and Game Boy Advance versions were released on October 28 in the United States,[2] November 14 in Europe,[21] and May 2004 in Japan.

[32] Eurogamer's Tom Bramwell concurred that "as a 'pick-up-and-play' sort of game, THUG is endlessly rewarding" and called it the best entry in the series.

Joe Rybicki of Official U.S. PlayStation Magazine said that, as an extreme-sports game, Underground has a real story with "honest-to-goodness characters".

[32] Ben Silverman of Game Revolution described the plot as a "silly" cross between those of the 1980s films North Shore and Gleaming the Cube, but he praised it for giving context to the level goals and keeping distance between the skill unlocks.

[34] By contrast, Game Informer's Justin Leeper thought that "it serves to make some of the less-entertaining goals tolerable, because there's a reason for doing them".

Bramwell called the graphics "unchanged and increasingly antiquated" and criticized the presence of "sharp, angular character models, eerily unrealistic lighting and odd little moments when the player is trying to turn round and ends up banging into a curb".

[4] Knutson, meanwhile, was positive regarding the graphics, art, and animation: he called them "nearly flawless" and praised the realism they brought to the inherently fantastical skateboarding genre.

[2] By contrast, IGN's Craig Harris praised the graphics of the Game Boy Advance version but was more mixed on the soundtrack.

Knutson lauded the high degree of customization; he summarized that "everything is expounded a hundred fold: from create-a-skater to create-a-park mode, it is simply amazing".

[32] GameSpy's Bryn Williams identified the level editor as an "extremely well-designed" feature that contributed to the overall "brilliance" of the full product.

[33][34] Reviewers for Famitsu magazine praised the story mode, whose open world format they compared to the Grand Theft Auto series.

[10] Despite his praise for the customization modes, Leeper admitted that his greatest enjoyment still came from "seeking great lines" and beating own scores.

[2] Similarly, Famitsu reviewers noted that, although the fast pace was extremely challenging at times, the high difficulty provides a sense of accomplishment when jumps and tricks are performed correctly.

[37] In Europe the week after the release, the PlayStation 2, Xbox, and GameCube versions were respectively the fifth, sixth, and eighth-best selling games for those consoles.

The gameplay, structure, and level design are very similar to those of Underground, but Underground 2 features new tricks and gameplay mechanics, like the Natas spin, the ability to plant customized stickers in levels with the "sticker slap", a slow-motion "Focus" mode, and the ability to earn points by having a tantrum after falling.