Although it's not an official name, everyone who works at the summit of Mauna Kea knows "Submillimeter Valley". It's the name given to the area that's home to all the submillimeter telescopes on the mountain: the CSO, the JCMT and the SMA. This region is a few hundred feet below the summit and the optical-infrared telescopes. I'm not entirely sure I know the reason why these observatories are in the valley rather than a little higher up where the other telescopes are, it may be a scientific one or purely logistical, or a combination of both. I haven't really thought about it before but now I have, I will ask and find out!
Submillimeter telescopes bridge the gap between observing in the optical/infrared and the radio regions of the electromagnetic spectrum. Unlike radio telescopes, they need a dry atmosphere in order to work hence the reason for placing them on high mountains such as Mauna Kea in Hawai`i or the high Atacama desert in northern Chile where most of the atmospheric water is below them. That's partly the reason you'll find infrared telescopes at similar sites as they are also affected by telluric water but often suffer a disadvantage: more often than not submillimeter telescopes can look right through high cirrus clouds as the water is frozen into ice particles and is undetectable at submillimeter wavelengths. On the other hand, ice shines rather brightly in the infrared so all we can do on those cloudy nights is detect cloud! I've spent many nights on Mauna Kea during which we can't work yet we get called by our colleagues at the JCMT telling us what a wonderful night it is!
The view above was taken just outside one of the mountain's infrared telescopes, UKIRT, looking down at Submillimeter Valley in May earlier this year. The shot below, from roughly the same place, was taken in March during one of the most awful few months of summit weather I've known in the last decade.