The highest energy particles are from the sky

These particles are produced somewhere in the universe after which some of them hit the Earth's atmosphere and few of those are detected by men. To still see a few of the highest energy cosmic rays requires very large observatories, such as the 3000 square kilometer Pierre Auger Observatory. Over the past century many remarkable facts on cosmic rays have been found. But for those at the highest energies the fundamental questions: What are. they?  Where they come from? and How they were produced?, offer a lively field of research.