I believe that the most unknown 400m+ structures in the world are the masts of the former North Sibirian CHAYKA-chain. At least two such masts were built: one at Tajmylyr, which was demolished in 2009 by explosives, after this CHAYKA-chain was shut-down ( video on
http://www.youtube.com/watch?v=UbPl4uPruoE&NR=1 ) an other at Dudinka (
http://ru-abandoned.livejournal.com/713371.html ). I believe, that of Inta (
http://www.wikimapia.org/#lat=65.966...=b&search=Inta ) may be of the same height, but I have no data.
Interestingly in the 1960ies for LORAN-C, which uses as CHAYKA 100 kHz as frequency and is very similiar to LORAN-C, 6 412 metres tall mast were built, situated on Hellissandur, Angissq, Cape Race, Iwo Jima, Marcus Island and Port Clarence. From these only that in Hellissandur still stands and is now used for broadcasting.
The masts at Angissq and Cape Race collapsed catastrophically. On Iwo Jima the mast first collapsed during construction and fell on the transmitter building, whereby several people were killed. The second mast was dismantled in 1993. On Marcus Island it was replaced by a smaller one in 1985 and that at Port Clearance was demolished in 2010 after USA terminated LORAN-C.
Although the 412 metres tall LORAN-C masts and the 460 metres tall CHAYKA-masts were built for the identical purpose, their antenna design is quiet different: all 412 metres tall LORAN-C masts were mast radiators insulated against ground. The 460 metres tall CHAYKA-masts are grounded and carry a rhombic cage antenna.
As in Northern Sibiria, there is permafrost, I believe their basements reach very deep into ground.
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