Mauna Kea is a 13803 foot (4200 meter) dormant volcano on the island of Hawaii. A number of observatories are located on or near its summit. The atmosphere is very dry, unpolluted and cloud cover is generally below the summit. It is also distant from cities.
The University of Hawaii built Mauna Kea's the first modern observatory with a 2.2 meter telescope in 1970. Currently, the land devoted to observatories is managed by the University of Hawaii as the Mauna Kea Observatories. Some current observatories:
There is concern about the mountain being overrun by telescopes, and the current plan is that some will be decommissioned when the TMT is completed, e.g., UKIRT.