It is a bit of cheating to include the described volcanoes of Kamchatka into the list of treks - strictly speaking these are all day hikes but the area is so remoteness and beauty of the area is certainly unique enough to mention it here. Kamchatka is a peninsula in the far eastern portion of Siberia/Russia. It is part of the "Ring of Fire" and features 28 active volcanoes along its eastern coastline (plus 1 to the west). Most of these volcanoes are pretty much inaccessible except by helicopter but some can be reached by 4x4s or trucks. The most popular include the Gorely, Mutnowsky, Avachinsky and Tolbachik volcanoes. The latter is located in central Kamchatka - about 550km from Petropavlosk, the only major city here, whereas the others are in the closer vicinity of the capital. Kamchatka has been a forbidden military zone till the early 90s and the touristic infrastructure inc. hotels, roads and information centers is miserable accordingly - expect unproportionally high costs for little service. Admittedly this is almost the end of the world here and there's only a 3 month season so this can't simply be a top value destination. Unless you've a decent skill level of Russian you probably have no chance but to join an organized tour to reach the trailheads.