The UnderGarden is a game set in an underwater world and it focuses on exploration. The player guides a creature through various underwater caverns. Starting from a main hub there are different environments that are unlocked gradually. There are a number of goals such as collecting special flowers or a hidden crystal, but these are not mandatory and all levels can be completed without collecting any of those. The "zen" experience comes first as the creature can bring back colour to the caverns to create a lush underwater garden.
To restore colour the creature has to collect pollen. By bumping sacks highlighted in green small sparkles of seed are collected and fill up a bar. Swimming in the vicinity of any plant then makes it grow in a wide variety of colours. There is no reward attached to the action, but it is the main game mechanic. There are also musicians that can be carried around to re-bloom flora to a different colour palette. Both pollen and musicians are dragged around by charging to generate a halo and the latter can be released when they are no longer needed. The creature can also boost to gain more speed.
Even though most areas can be crossed without many obstacles, there is a small number of puzzles. Certain entrances are blocked by rocks or levers. The player has to locate special plants that generate fruit that can be carried around as well. There are yellow flowers that press objects down, blue ones that raise objects, and also red plants that provide bombs to blow up rocks. Later levels introduce powerful currents that steer the character in a certain directions and blue balls that drain all collected pollen. The different levels are connected through warp zones. There are also checkpoints; when stuck you can warp back to the latest one. There are no opponents, there is no health system and the creature cannot die. The game can be played cooperatively with another creature helping to bring back colour.