I have recently played a beta of an in-development map pack for Doom called Vela Pax. The design philosophy is basically "six large levels", it has been in development since 2010 I believe.
So I thought "cool, I'll check it out". I started the first level and realised that there were 1082 monsters in the level... I then looked at the map.
This is
ridiculous
The map took about an hour and a half to finish. This was
level 1. This was developed by
one guy.
The amount of dedication to create something of this scale has to be applauded. The map was pretty fun too, though navigating it wasn't exactly easy.