Like its predecessors in the Tony Hawk's series, Underground is centered on skateboarding in a series of levels.  The player performs tricks via combinations of analog stick and button inputs. For example, the player initiates an ollie (a jump trick) by holding and releasing the jump button. During an ollie, the player may change the position of the analog stick and press one of two buttons to perform either a flip trick (such as an impossible or kickflip) or a grab trick (such as a benihana or nosegrab). The player can grind on certain edges and rails; different tricks may be performed during a grind based on the position of the analog stick. On quarter pipes, the player may execute lip tricks. While skating on flat surfaces, the player may manual in multiple ways via button combinations. Miscellaneous tricks include acid drops and wall-rides.
While a grind, lip, or manual trick is underway, a balance meter appears: unless the player prevents this meter from falling to the left or right, the character will bail and need a few seconds to recover. Bailing can also be caused by falling without one's board facing downward. Completing tricks in succession without bailing is called a combo.  Comboing raises the player's score and fills up the Special Meter; when it is full, the player is granted access to more elaborate tricks worth more points, such as the McTwist and 540 Flip.  Underground introduces the ability to dismount one's skateboard, which allows the player to explore levels more carefully and reach new areas. Each level features at least one vehicle, usually a car, that the player can drive.
The custom skater performs a 360 Varial Heelflip Lien as an objective in Moscow
The levels are based on regions of the United States and other countries.  In each level, certain tasks that advance the game's narrative must be completed before the player can move on. These tasks include score attacks, races, item collection, and reaching one of many gaps[b] found in a level.  Each level houses one professional skateboarder, who provides a sidequest that unlocks a trick for the Special Meter.  On account of the levels' large sizes and the integration of goals into the story, Underground has been described as an adventure game.  Characters can level up their stats—which include jump height and speed—by completing optional goals in a level;[4] this adds an element of role-playing gameplay.  Other gameplay modes include multiplayer minigames—one, a combat mode called "Firefight", can be played online in the PlayStation 2 version of the game—and a "free skate" mode that lets the player explore levels with no goals or story.
Underground features extensive customization. The player creates a custom character for the story mode, and may not play as a pre-made professional skater outside a few contexts: a special scene late in the game, minigames, and the free skate mode. A level editor allows the player to create skate parks with a large array of objects, ranging from traditional skate park elements like halfpipes, ramps, funboxes, and grind rails to more outlandish pieces like buildings and sections of elevated freeways.  The player can change their park's time of day and environmental theme. Tricks, skateboards, and level goals may be customized as well