A unique first-person shooter, Submarine Secrets primarily takes place in Combine’s submarine. After receiving the assignment from Alyx, Player must enter the submarine, find and retrieve important documents and fight combatants. Not only does Player have to fight to stay alive while finding a way out of the submarine but, the Player also has to sink the submarine while escaping with the documents. Failure to exit the submarine in time will result in death by drowning.
To create more challenge to the game, a time element of rising water exists to keep the Player moving through the submarine quickly.
Once the Player retrieves the documents, a way must be found to sink the submarine. The Player starts a water leak and the submarine progressively fills with water as the Player fights and attempts to discover a way out of the emergency lit passageways.