A new cosmic ray observatory being built in Utah - 'the Telescope Array'. Works on the same principle as many others already built (e.g. Fly Eye), Cherenkov radiation* emitted by the charged particle rays as they encounter a medium where the speed of light is slower than the one they were in (atmosphere vs space). They're looking for CRs beyond the GKZ limit*, and to do that you need a huge array. Will take four years to build. There's been limited data to date on whether there are CRs beyond the GKZ.
For more on the whole area of high-energy astrophysics, see the Stecker paper (it's in another thread somewhere here - really interesting stuff!)