Abandoned - A nicely designed, simply titled English-language collection of photos of abandoned industrial locations around Moscow (in English).
ACIS - A small, official-looking page from an entity styling itself as the Artificial Caves Investigation System (in Russian).
Analyser - Stories and pictures from an avid caver and tunneller (in Russian).
Deepdig - Site of a relatively new digger group featuring news, expedition stories, maps and photos (in Russian).
Deserted World - Text and photos related to delicious abandonments and tunnels around and under Moscow (in Russian).
Diggers.ru - Main site of the Moscow State University Diggers (in Russian).
DigPit - I suspect that if I could read this site I would be able to determine conclusively that it sucks; as it stands, that's just my best guess (in Russian).
Docent - Moscow-based site featuries articles, galleries and more, related to subway tunnels and other underground places (in Russian).
DIGzone - Design-laden site featuring some photos of transit tunnels and abandoned industrial sites in Moscow and Kursk (in Russian).
Euthanasia - Black-and-white photos of some very bleak spaces, including both industries and former residences (in Russian).
Gugno Diggers - Illustrated accounts of draining and tunnelling expeditions, as well as related jokes, dreams and fantasies (in Russian).
Gulo - A small collection of photographs, FAQs and other text files related to tunnels under Russia (in Russian).
No Name Digger Cru - Site of a group of shy explorers operating in the area under Kiev, featuring tales, photos, member profiles and a message board (in Russian; currently down, hopefully temporarily).
North-west Diggers - Extremely large and impressive site, spanning multiple domain names and featuring message boards and huge photo collections of subterranean sites (in Russian).
Novisibirsk Diggers Speleopage - Tunnelling and caving in the frozen land of Siberia (in English and Russian).
Podzemka Diggers - Only half of this site is broken. The other half is under construction (in Russian).
Pripyat Ghost Town - A woman's accounts of her rides through, and explorations of, a radioactive town abandoned in the wake of the Chernobyl disaster (in English).
Sevdig - Home base of the Sevastopol Diggers, featuring nice pictures and more incomprehensible text than you ever dreamed of (in Russian).
Singl Wolf'a - This site features tips, an FAQ, and photos of rooftops, abandoned buildings and underground sites around Moscow (in Russian; currently down, hopefully temporarily).
Twilight Zone - Extensive photo galleries of rooftops and abandoned places (mostly in Russian, with a small English section).
UnderMinsk - The very cleverly designed site of a group of diggers under the Belorussian capital (in Russian).
Urban Moscow - A nicely designed picture gallery focusing on abandoned buildings in Moscow; on an unstable server (in Russian).
Urban Trip - A photographic study in the aesthetics of rust that bravely probes beyond the signs reading "passage is forbidden" (in Russian).
Vladivostok Diggers Club - Group site site featuring explorations of all things subterranean in the Russian Far East (in Russian with a small English section).
Vladivostok Fortress - Abandoned military leftovers in the Russian Far East (in Russian with a small English section).