The summit of Mauna Kea, Hawaii, will be home to the world's largest telescope. The Thirty Meter Telescope (TMT) will be far larger than any of the 13 existing telescopes on the extinct volcano. The twin 10-meter Keck telescopes are currently the largest on Mauna Kea, an attractive site since it boasts clear skies 300 days of the year. TMT is expected to begin official observations in 2021.