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The engine is PlayCanvas. It's an open source game engine. Check out the repo on GitHub here: https://github.com/playcanvas/engine There's high level editor tools (kind of a blend of Google Docs and Unity) here: https://playcanvas.com/

Interesting...so does it only work on modern browsers?

Yes. Most importantly it uses WebGL, which isn't available on any versions of IE before IE11: http://caniuse.com/#feat=webgl

can I contribute to this game or fork it? God I can't stop playing it!

You can indeed, feel free to do so. Here is source of server side code: https://github.com/Maksims/tanx And here is client side: https://playcanvas.com/project/45093/overview/tanx And my email if you want to contact directly: core[at]moka.co