Thirty Meter Telescope

The Thirty Meter Telescope is an under-construction extremely large telescope that has become controversial due to its planned location on Mauna Kea, on the island of Hawaii. The TMT would become the largest visible-light telescope on Mauna Kea.Scientists have been considering ELTs since the mid 1980s. In 2000, astronomers considered the possibility of a telescope with a light-gathering mirror larger than 20 meters in diameter.

Source: Wikipedia