Submachine Universe

Submachine Universe is probably the first ever MSO (massively singleplayer online) created in 2010, extended ever since. You can freely travel between different locations, read theories about Submachine and observe how things work inside this network. There are locations that you might recognize from previous games, there's a lot of new yet undiscovered content, there are even locations that you think you know, but they're somehow different. Surely worth a look.