In Boston.com there was a beautiful photo spread of the Large Hadron Collider.
One of the photos looked like this:
(Maximilien Brice; Claudia Marcelloni, © CERN)
There is a curious sign at the top of the tunnel that looks like this:
In the article they say, "The sign at top warns of the presence of helium, argon and/or nitrogen in nearby pipes - gases that (if they leaked out) could displace oxygen and cause unconsciousness."
But I think it means the following: