Alexander Friend walks up to a stainless steel door, twists some latches holding it into the wall, and lifts the 7-kg rectangle out of its hole and onto the floor, revealing a sideways 152 x 101 cm window into the earth. This is window 17 of the 24 in the US Department of Agriculture Forest Service's Houghton, Michigan rhizotron - an underground lab named for the Greek word for root. Like the others, window 17 is divided into 15 panels that can be removed easily to take samples, inject dye, or whatever else a scientist might think of. Today, there's a centipede making its way by one of the panels, as well as scattered roots, probably from a nearby sugar maple.
Northern Michigan isn't known for its temperate winters, but the soil in the Houghton region doesn't freeze because of the constant snowpack. So on a December day, there's plenty ...
