An interesting art project that seems more an interactive-screensaver than an actual game, and feels severely lacking in substance or real entertainment value.
In David O'Reilly’s debut gaming project, Mountain, one watches over the titular landform as it floats through the infinite void, apropos of nothing. There is little interaction to be had apart from using one’s keyboard keys to plink out a handful of musical notes, observing roughly two dozen random objects collide with, and become partially embedded inside, your mountain at irregular intervals, and reading from a selection of generically upbeat fortune cookie-style messages which fade on-and-off the screen more frequently the longer you “play”. 
Mountain would have worked much better, in my view, as an independently-released shareware with a donate-if-you-want-to button as implemented (quite successfully) by many indie developers, especially first-timers. Attached to the game, presently, is thirty-one Steam Achievements which reportedly take hundreds if not thousands of hours to meet the requirements of legitimately, which is just a pointless joke in my eyes.
This game is not recommendable to the large majority of Steams users, who would quite justifiably not want to spend more than an hour, maximum, with the Mountain, but if you really want a desktop zen toy and nothing more and don’t mind the price, this is an option.