OBELISK When a bunch of engineers, scholars, and craftsmen come together to figure out how Egypt's massive obelisks were erected, there's bound to be some disagreement. By today's standards, the feat seems impossible- in creating these mighty monoliths to the gods, the pharaohs' teams had nothing to work with but stones, ropes, logs, and dirt. Using such simple tools- and superhuman patience- ancient workers carved from solid granite, one of the hardest rocks to work, a stupendous shaft weighing some 400 tons. Then they chiseled, without benefit of a chisel, exquisitely detailed hieroglyphs along its side. After transporting it miles down the Nile to its base in front of a temple, their crowing achievement was setting this colossal object upright without a fracture line in sight. How did they do it? NOVA puts Egyptologist Mark Lehner, stonemason Roger Hopkins (of This Old House), and other experts to the challenge, testing out various theories involving a temporary sandbox, a giant counterweight, a gradually built-up ramp, a slowly sliding sled, levers, pulleys, ropes- and the giddy enthusiasm of hundreds of local helpers who lend their monumental efforts to the task. PYRAMID Towering over the Giza plateau, the Great Pyramid stands in testimony to one of the world's most unfathomable feats of engineering- and cooperation. Built over 4,000 years ago, the stupendous structure marks the effort of tens of thousands of workers who, over some 23 years, cut, raised, and precisely placed its two million blocks of heavy stone. Looming over Egyptologist Mark Lehner, stonemason Roger Hopkins (of This Old House), and a team of Egyptian workers is a daunting challenge: to replicate this achievement (albeit on a small fraction of the scale), while putting some pyramid-construction theories to the test. How did the pharaoh's builders align the structure's sides to the compass points? were its blocks dragged up on ramps? Inched up by levers? And if the NOVA gang gives all these hypotheses a try, will they ever finish an 18-foot pyramid before their three-week deadline is up? "I don't think there are any huge mysteries about the nuts and bolts of how they made a pyramid," concludes Lehner. "But what caused them to do it all of a sudden? What motivated them to do that? That's the real mystery.