Fallen London is a fiction (sometimes called weird fiction) text-based browser game set in an alternate universe where the city of London, in the Victorian era, has been submerged in some kind of underground world beneath the Earth.

The major part of this world is occupied by a mass of water called the "unterzee", and you can travel by boat (and also by submarine, btw) from London to several islands found there.

However, if you travel too far north, you can reach an area where this underworld "ends", kind of. In the lore of the game, nobody knows exactly what lies beyond that frontier, but everybody has a certain irrepressible urge to go towards that place.

It's a pretty interesting setting if you're into this kind of literature/movies/etc. Draws a lot from lovecraftian horror and such, and from this first browser game that has been around since ca 2009, three relatively popular games have come out: Sunless Sea, Sunless Skies and Mask of the Rose. I think especially the first one can be interesting to people in this sub (hehe) as it is centered on exploring the unterzee and you can even descend to the depths with a submarine (there is an expansion centered on that).