Similar to its predecessor, Mercury Meltdown is a puzzle-platform game. The goal is to navigate one or more blobs of mercury to one or more finish posts in the level by tilting the stage using the analog stick of the PSP. Players automatically lose the stage if all of the mercury is from the stage is lost, or no longer meeting the requirements to complete the level. The mercury can be split into multiple blobs by using sharp objects or obstacles. The color of the mercury can be changed using a Paintshop or merging with other blobs of different colors. Color mixing is based on the RGB color model.[1][2] The game is made up of worlds referred to as "Laboratories" that are split into 16 stages represented as test-tubes. Stages have achievements for completing with 100% remaining mercury, obtaining the top score, and obtaining all bonus stars. An additional achievement is granted for obtaining all three in a single stage and can all be obtained individually via multiple playthroughs. Laboratories are unlocked after accumulating enough mercury after each stage completion. If players do exceptionally well in one particular laboratory, a secret 17th stage is unlocked.[1][2]
Mercury Meltdown introduces the Playground; a circular arena, with most of the items found in stages to play and test with. Another new mechanic is the ability to change the mercury into 3 new states: Cold, Hot, and Solid. The Hot state makes the mercury an easily splittable liquid that travels quickly. The Cold state makes the mercury a semi-solid blob that moves slowly and is harder to split. The Solid state turns the mercury into a solid ball that can't be split, allowing it to traverse over rails.[1] Multiplayer is accessible between two PSPs via Ad-Hoc wireless mode or online network infrastructure mode. In multiplayer mode, players can participate in battle mode in which players can race each other from previously unlocked single player levels. Bonus stars are replaced with battle pick-ups that can assist players or hinder their opponent.[1]
In addition to the main game, Mercury Meltdown introduces five unlockable party games: Rodeo, Race, Metrix, Shove, and Paint. In Rodeo, players tilt the stage to prevent the mercury from falling off. In Race, players race mercury around a track. Metrix is a match 3 puzzle minigame requiring one to make a group of three or more colored blobs that fits inside a pre-defined grid. In Shove, players aim the mercury for the center spot of a target, avoiding hazards; similar to curling. In Paint, players move the mercury to paint the tray in their respected colors before the opponent does. These can be unlocked by collecting the bonus stars in the main game. All the party games can be played in single-player and multiplayer