Sunwatcher

 was a base set up by humans on Mercury to monitor the Sun. It was a domed base that moved on tracks with the orbit of Mercury so that the base was always located at the optimum temperature. It was powered by three solar pylons.

Three months after it was established, the base was staffed by five scientists. They encountered a native Mercurial who tried to warn the scientists of three "giants" that had attacked his people. With the Second Doctor's help, they learned that the "giants" were the solar pylons. (PROSE: Mercury)