Underneath a relict patch of illinois prairie, complete with a small herd of grazing buffalo, protons and antiprotons whiz along in opposite paths around a four-mile-long tunnel. And every second, hundreds of thousands of them slam together in a burst of obscure particles. It’s another day at the Tevatron, a particle accelerator embedded in the verdant grounds of the 6,800-acre Fermi National Accelerator Laboratory complex in Batavia, about 50 miles due west of Chicago. There have been many days like this one, some routine, some spectacular; of the 17 fundamental particles that physicists believe constitute all the ordinary matter and energy in the universe, three were discovered here. But there won’t be many more such days. By October 1 the power supplies for more than 1,000 liquid-helium-cooled superconducting magnets will have been turned off forever, the last feeble stream of particles absorbed by a metal target, ending the 28-year run of what was until recently the most powerful particle accelerator in the world.