Supertall is a colloquial, recently-coined term that refers to an extremely tall skyscraper. It is especially common jargon among skyscraper-enthusiast bloggers.
Although no official definition exists, a height of approximately 300 meters or 1,000 feet is a generally agreed minimum of architecture enthusiasts for a skyscraper to qualify as supertall. The term also applies to proposed structures over 1 kilometer (1,000 m / 3,281 ft) or 1 mile (5,280 ft / 1,609 m) in height — designs which have never advanced beyond the concept or planning stages. Additionally, the term may be used for an architectural structure of any type taller than 300 meters, such as free-standing towers, guyed masts, chimneys, bridge pillars or other non-building structures. Most supertall structures rank among the top 10 in a particular category of (local) height records.
Of the approximately 1,150 existing supertall structures, most are guyed masts for FM radio and television broadcasting in the United States. This stems from the country having expansive flat areas where individual TV stations can build towers to service a wide area or population center, and of the circumstance that in the USA, most broadcasting companies built their own tower; therefore it is not rare for multiple supertall radio towers to be located in close proximity. Other supertall structures are guyed masts for TV-broadcasting outside the USA, for scientific experiments and for transmission in the VLF, longwave and mediumwave range, free-standing and partially guyed TV-towers, chimneys and last but not least some skyscrapers. There are also two supertall electricity pylons belonging to Yangtze River Crossing, a supertall bridge (Millau Viaduct) and a supertall dam (Nurek Dam).

Challenges
Supertall skyscrapers save land by housing a large number of residents and/or workers on a relatively small footprint. However, fire safety and rapid evacuation problems increase with building height and the number of people the building is designed to serve. In addition, water and sewage networks become more expansive as buildings get taller, especially when they must handle the needs of many people. Many skyscrapers have observation decks open to tourists.

Buildings
Supertall radio masts and towers for UHF-/VHF-transmission allow a wide area of reception. Because of reflections, which disturb radio wave propagation, their erection is only sensible in flatter areas, without higher mountains.
Therefore nearly all very high radio towers (taller than 300 meters (1,000 feet)) for UHF-/VHF-transmission are situated in flat areas. As guyed masts of same height are cheaper than free-standing towers, building guyed masts is preferred in most cases. Free-standing towers, however, show less oscillation by external forces like wind than guyed masts, and require no sites for guy anchors. So they are built either if no place for guy anchors is available or for towers which may not show too large oscillations by external forces, as towers with observation decks and/or towers with larger cabinets and/or directional antennas.
In some cases building a partially guyed tower, consisting of a free-standing basement tower with a mast guyed to the ground on the top, may be a good choice under technical and financial points of view, as it is cheaper to build than a completely free-standing tower of same height and offers a basement section more stable than a free-standing tower.
Supertall mast radiators, e.g. half-wave radiators for longwave radio, allow larger areas of fading-free reception, because in comparison to shorter mast radiators, the ratio of skywave and groundwave radiation is much smaller, so fading occurs in areas much further away from the transmission tower than using a shorter mast radiator. However, because nowadays the range of longwave transmitters is more determined by jamming from other stations working in the same channel, using halfwave mast-radiators does not greatly improve longwave reception at a greater distance from the transmission antenna. The only realized half-wave mast radiator for longwave was the Warsaw Radio Mast. The usage of supertall mast radiators and antenna structures with heights between 300 and 450 meters (1,000 and 1,480 feet) is however very sensitive for high power longwave radio stations with greater bandwidth requirement like longwave broadcasting or LORAN-C, because these antenna structures require for the realization of a quarter-wave length radiator no or only little electrical lengthening, which reduces bandwidth and antenna efficiency and increases also antenna voltage.
For VLF transmission the use of supertall masts either as mast radiators (electrically lengthed as an umbrella antenna) or as carriers of wire antennas is perhaps the best choice. However the erection costs grow faster than the height of the masts, as do the costs of maintenance. So one tries to find out whether it is possible (by using electrical lengthing) not to build the masts unnecessarily tall. Because the bandwidth of most VLF-transmitters is very low, the use of excessive electrical lengthing is possible without bigger problems. At VLF there is also very little skywave propagation, so there is no requirement for skywave suppression characteristics of the antenna. In contrast to its skywave propagation, the groundwave propagation of VLF is very good and one can easily increase the range of the transmitter by increasing the power. In this frequency range it may be cheaper to run a high power transmitter with a less effective antenna using lower masts than to build and maintain an antenna using extreme tall masts. So VLF transmitters often use very tall masts (height between 200 and 400 meters (660 to 1,320 feet)) for their transmitting antennas, but their masts are not among the tallest ever built. In the Western hemisphere the masts of VLF transmitter Lualualei on Hawaii are the tallest structures used for transmission in this frequency range.
Nevertheless satisfactorily working VLF and LF transmitters can be built according to the works of the radio engineer Ernst Alexanderson without using supertall towers. Such antennas include the Kalundborg longwave broadcasting transmitter and the SAQ-VLF transmitter in Grimeton, Sweden. Their towers are approximately 120 meters (400 feet) tall.
For mediumwave-transmission, supertall mast radiators are a bad choice because they show high skywave emission, which results in excessive fading at night. However there are some special highly effective fading-reducing antenna types for mediumwave broadcasting which do require supertall towers. Because of the high erection costs for the required towers, their use is only sensible for high power stations working on nearly interference free channels at the lower end of the mediumwave band. So only a few broadcasting stations like RKS Liblice 2 use or have used such antennas.

Radio towers
Historically, supertall chimneys were used to improve the dispersion of waste gases and heat from a factory or power generation station. However, tall chimneys did not eliminate toxic substances in the smoke — it simply distributed a lower concentration of pollutants over an increased area, often over a population center. A well known exemplar is the Inco Superstack in Sudbury, Ontario. In many countries, recent enactment of stricter pollution laws has reduced simply dispersing exhaust with huge chimneys and requires emissions cleaning equipment to be installed. The regulations reduced the need for extremely tall chimneys, as the resulting polluting emissions are of a low concentration so that distribution over a wide area is no longer necessary. However, a supertall chimney may be appropriate for a factory or conventional thermal power station situated in a valley, exampled by the Trbovlje Chimney at Trbovlje Power Station in Slovenia, located in a deep valley.

Chimneys
For power line crossings of sea narrows and wide rivers overhead powerlines carried by tall pylons on each side of the sea narrow or river can be used. While such crossings with pylons lower than 230 m (755 ft) at each shore of the sea narrow or river were reallized in many countries, only one such crossing with supertall pylons, the Yangtze River Crossing in China, was built. This results in the fact, that if such tall pylons would be required, laying an underground cable through the sea narrow or the river may be more practical. At long spans, the use of bundled conductors, which are standard for powerlines with voltages greater than 200,000 volts (200 kV) for physical reasons, can result in wind-induced oscillations, that may even lead to short circuits, if they approach too closely. Single conductors thick enough to transport the same amount of electricity as bundled conductors show more corona losses and are therefore not the best choice, but sometimes used for overhead powerline crossings with not extremely high power loads. A more serious problem at such crossings, is that conductive materials of high tensile strength are required, if crossing pylons should get not too high ( the minimum clearance of conductors of upper mentioned Yangtze River Crossing using standard conductors with a span width of 2,380 m (7,808 ft) is 55 m (180 ft), while the pylons used are 346 m (1,135 ft) tall!). Unfortunately these materials demonstrate a higher specific electrical resistance than standard conductors, resulting in higher losses. Therefore their use is only sensible, when power transmission requirements are not too high. It is hereby to mention, that the overhead powerline crossing of Messina Strait with steel conductors, two 232 m (761 ft) tall pylons on each side of Messina Strait and a span width of 3,280 m (10,761 ft) was replaced by a submarine cable.

Bridges
Supertall dams require a deep canyon-like valley as a construction site. So far only one such dam (Nurek Dam in Tajikistan) has been built and two further are under construction.
Solar updraft towers in contrast have to be built as supertall structures, because this improves the efficiency of the facility.
A future use for supertall structures may be special towers with maglev tracks for launching spacecraft and space elevators.