The fact that there at three installations in Alaska and one in Puerto Rico, for one thing. There are also installations (under other auspices, but using some or all of the same technology) in northern Norway near Tromsø; Japan; the University of Leicester; Sodankylä, Finland; Jicamarca, Peru; near Moscow, Nizhny Novgorod (SURA) and Apatity, Russia; near Kharkov, Ukraine; and in Dushanbe, Tadzhikistan.

It took VERY little searching to turn this information up -- and it's not classified. In fact, the US HAARP installation near Tok, Alaska has had open houses for the public.