Oh no, this feature has gone extinct, meaning that it isn't in the game anymore! Please don't remove any content from this page or this Lemming will eat you! :(
Sandbox is a removed game mode, it was first added on Jan 28th, but was first mentioned in Feb. 1st in 2017. It was added in Feb. 4th, 3 days later. In sandbox, you can instantly evolve to the next tier by pressing the up arrow and devolve by pressing the down key. There were currently 11 servers. You can join in by clicking the big sandbox button, to the bottom right of the logo. The Leaderboard players are listed in kills instead of XP.
If you are a Top Tier animal like Dragon and you press the up key then you will devolve to the mouse
Since Feb 4th you now don't get burn when you double click but you get frozen when double pressing left-click, for an unknown reason.
Before February 12, you could upgrade with the double click.
It used to be accessible by typing mope.io/sandbox into the search-bar. This was the same as currently what sandbox is but only had 1 server, making it so there was a LOT of players.
If you try to go the sandbox now (by typing mope.io/sandbox/, 4th February), it will show a blank page with only the following message: "You sneaky (and smart) person, mope.io Sandbox isn't officially live yet, but is being improved and is coming soon! -Stan", the Mouse skin, and a link to the normal game. This page can still be accessed.