For the implementation of the VLF or Very Low Frequency-Navigation System “OMEGA” in 1976, the US Coast Guard erected a 417 meter tall radio tower, which is taller than any man-made object existing today in the European Union!
Built near Paynesville in Liberia, it served together with seven other stations as part of a worldwide navigation system, and which could be used to communicate with submerged submarines. While VLF has a very low data transmission rate, one of the advantages of VLF is that, unlike higher frequencies, of say VHF or UHF, it can deeply penetrate seawater to reach submarines. (Frequencies such as VLF and ELF, extremely low frequency, have been cited as the casue behind disoriented and injured whales who communicate using the same wavelengths) However by the 1990s, the global satellite system and GPS made OMEGA obsolete and so OMEGA was shut-off on September 30th, 1997 and the station was given back to the government of Liberia.
Unfortunately during this period the government of Liberia was involved in a bloody civil war and had no use for the transmitter and the huge mast, which though it survived the civil war until today, continues to go unused. As there is no access restriction to the mast, it is theoretically possible it could be used for skydiving, powerful transmission experiments and Tesla experiments.
For those Tesla experiments however, one ought not forget, that Liberia is a country with a very unreliable electric power supply, so one should come with their own, very large, power generator.
Update: The Paynesville tower was demolished in 2011.