Go! Underground (地底へドンドン！) is a short 2D platformer from Japanese game developer Dot Zo Games. The game puts you in the shoes of an archeologist searching for ancient artefacts in an underground cave complex. Only a few treasure chests are exposed from the start; the rest can be found by solving puzzles or performing certain actions. Thankfully, the treasure info screen (accessed with the A key by default) provides you with hints on how to find everything.
The signs you can read in-game explain the game's mechanics by means of icons, so you don't even have to know Japanese to understand what to do. Controls can be viewed and changed under the settings menu at the top of the window.

The game has charming pixel art graphics and nice music, and shouldn't take more than an hour to complete. To download, go here and click the yellow button. Please note that Windows is required, as well as a minimum display resolution of 1280x720.