Great tile improvement. Can be constructed by a Great Prophet that has all its uses of "Spread Religion" remaining.
Holy Sites might not look so useful at first glance, but in the hands of a skilled player they can turn into a great boon. If your civilization adopts the Piety policy tree, they turn into an economical powerhouse, adding +3 Gold and Culture to their potential. Because such civilizations are more focused on Religion anyway, and may use Faith for more things, these Great tile improvements become very important to them.
A place of religious significance, holy sites are locations held sacred as a testament to the gods, prophets, and traditions of a given religion. Often the destination for religious pilgrims, these sites are often visited by the faithful in the hopes of strengthening their connection to God, while also paying their respects.