SciAm frequent contributor Charles Q. Choi writes from the Yukon on an expedition with researchers from the American Museum of Natural History in New York City. Their goal: to recover intact DNA from mammoths, which once roamed the tundra but went extinct some 11,000 years ago. To read Choi's previous post click here. DAY 3: June 15, 2008 We have gone prospecting today, driving some 75 kilometers (47 miles) away from Dawson and deeper into the Klondike forest, passing snow and beavers. Equipment rattles in the back of the car as we go over bumps in the rough gravel roads. Ears pop as we ascend to range across the tops of Yukon mountain ridges, ravens gyring overhead. We first stop at the largest operating gold mine in the Klondike, Ross Mine. This is a placer mine, meaning that the gold is hidden in sand and gravel deposited by modern or ancient streams or glaciers"”the panning for gold carried out during the California and Klondike gold rushes is an example of placer mining. The mine manager shows us mammoth tusks they found, with full tusks ranging up to 8 to 12 feet long, 8 to 10 inches in diameter, and close to 200 lbs. in weight. We then scout out sites of future digs with him in Ross Mine's open pits, each the size of at least a dozen football fields. Huge bulldozers and excavators rumble in the background. Scientists put a lot of energy into excavation and then can spend months in the lab poring over collected samples, so they want to make sure they get the very best samples. The researchers walk around scanning the exposed strata in the pit walls to mentally reconstruct the history of the locale, using their ice axes to dig up and inspect Pleistocene soils. One hope is to find evidence that might help solve the mystery of why so many megafauna went extinct in the Americas"”mammoths, giant sloths and the like. * When it comes to fieldwork in rough terrain, one of the perils is car trouble. After we leave Ross Mine to drive to a creek to prospect for bones amidst shoulder-high willows, moose tracks and the hum of mosquitoes"”we end up uncovering a dozen or so bison fossils that stream water had exposed"”we find that the rental SUV that Ross MacPhee, Clare Flemming, Lee Arnold and I drove in had a flat tire. After figuring out where the rental agency hid the jack and instruction manual, the scientists change the flat admirably quickly. Lunch seems to be the perfect reward for the job just completed, and we enjoy avocado, Brie, bagels, English muffins, raspberry jam and yogurt-covered granola bars. Of course, the cosmic joke is that after lunch, a tire goes soft in the SUV that Duane Froese and Svetlana Kuzmina are in. Time to get the jack and tire iron out again. VIDEO: fixing the flat tire

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-- Edited by Christie Nicholson at 06/23/2008 4:38 PM