Subjective.
If a space, tomb, church, structure, well, tree or whatever is dedicated to a god, spirit, ancestor or whoever by builders, users or whoever - then it is sacred to them. A mosque is sacred to a Muslim, but not to me as a non-believer. I should still respect its sacredness although I don't share or subscribe to it. Does any atheist on this list use the word sacred personally other than relating to second or third persons? If you have no god or religion how can anything be sacred to it? Can you have a sacred vacuum? For an atheist, it only works if you carelessly change the meaning of the word "sacred" to mean something else. People do of course and when they say that something is sacred to them, they often really mean special, spooky, inviolate, precious etc etc. "Wembley - the sacred turf" comes to mind, but then we have already covered that ground.