 Cosmic rays were discovered in 1912 by a man called Victor Hess who took a balloon high up into the atmosphere and then in 1938 a man called Pierre Auger, a French physicist, discovered by putting Geiger counters a few meters apart and eventually 300 meters apart that there were huge showers of particles coming down in the atmosphere. Cosmic rays are the nuclei of the common elements like hydrogen, helium, iron that have been accelerated to high energy somewhere in the universe. They are continuously bombarding our atmosphere and some of them reach us at sea level a million go through your body every night. One of the fascinations about this is how nature organizes itself to produce particles of that energy. One of the really really interesting questions that we're trying to answer is how and where are these particles created. I'm standing on little arm scruff looking at the site of Havera Park. The rock is actually inside the area and you can see over there where the mast is what was the center of our laboratory. Well the detectors that we used to observe the cosmic rays and to observe the showers were large tanks of water. The water was looked at by a device called a photomultiplier and we picked up Chernkov light which is produced when particles move through a medium with a speed which is greater than the speed of light in that medium. When we finished we believed that there was one particle above 10 to the 20 electron volts landing on a square kilometer every century. We now think the number is maybe a few per square kilometer per millennium. In 1990 I wrote a paper saying that we really needed to build a detector of a thousand square kilometers if we were going to make some progress and this idea was picked up by my friend Jim Cronin and when I met him the following year at a conference the first words he said to me was you're not ambitious enough we're going to build five thousand square kilometers and eventually we did that at least three thousand but in Argentina. Well the OGA collaboration has around 400 members. There's 18 countries involved typically about 400 people sign our scientific papers. The key features are large tanks of water which are on a grid with a spacing of 1.5 kilometers. They're very like the detectors that have a park and there are four fluorescence detectors which overlook the water Chernkov detectors and when it's dark and there's no moon we get coincidences between the two and this allows us to make some very very accurate measurements. One thing that many people have been searching for since 1966 was a decrease in the number of particles at the highest of energies. That had been predicted shortly after the discovery of the microwave background radiation. Microwave radiation provides a sort of shield around the earth a shield that is a hundred megaparsecs thick and it prevents particles of very high energy reaching us from larger distances. We've found a correlation at about the 40% level with active galactic nuclei which are very very powerful galaxies with massive black holes at the center and at the moment it looks as if Centaurus A which is a powerful radio galaxy only four megaparsecs away from us it may be the source of a lot of our higher energy particles. The great thing about working in cosmic rays is that you have to know some particle physics, you have to know some astrophysics, you have to know a lot about quite smart technologies and putting those together has been really great.