I think you're thinking of the Rockey Mountain Arsenal, which is now a wildlife preserve just outside Denver. It had been one of the largest chemical weapon repositories. When they started getting rid of these chemical weapons, they decided the best solution would be to pump them deep underground. Why they thought that was a good idea, I do not know. It reportedly caused earthquakes.
Here's the Wikipedia entry, it's pretty fascinating: http://en.wikipedia.org/wiki/Rocky_Mountain_Arsenal

