3000km^2 in Argentina: the Pierre Auger Observatory (http://auger.org) for ultra high energy cosmic rays.
It was meant to get a follow-up 7x the size in Colorado but it was defunded by NSF as a result of the 2008 crisis.
The exciting prospect of this was that at extremely high energies of cosmic rays (where they get very sparse: 1/km^2/century), these charged particles have a relatively short range (in astronomical terms - they're not from our galaxy; look up the GZK effect). That meant that they wouldn't be deflected by magnetic fields as much and could - thus the intent - be traced back to their sources. The clou: we have no credible theory for any process that would put this much energy into a single particle, so knowing their sources would be a step towards solving a physics mystery.
Sure I think we’re already doing that, but I also think this makes it very difficult to observe results let alone make controlled experiments. I can’t recall any particle or fundamental force that was validated / found in this way, the only thing I can think of are tests of the theory of general relativity that use astronomical observations, but I don’t recall anything in the field of particle physics (it’s not my field though so I might just have missed it).
Going much further in the past, you have the discovery of the element helium, whose existence was predicted by the periodic table, and it was detected first in the sun atmosphere (looking at the spectrum of sunlight during an eclipse) than on earth (where it is very abundant, but mixed with natural gas).
The hunt for dark matter, for whatever particle/string/widget is involved, is a particle physics problem. String theory may find support, or not, through the observation of black holes (ie fuzzball).
Neutrino detectors made big contributions in thier day.
And telescopes to study objects like quazars that too are just sitting there demoing high energy physics 24/7.