An ancient slab of Earth's crust buried deep beneath the Midwest is sucking huge swatches of present-day's North American crust down into the mantle, researchers say. The slab's pull has created giant "drips" that hang from the underside of the continent down to about 400 miles (640 kilometers) deep inside the mantle, according to a new study. These drips are located beneath an area spanning from Michigan to Nebraska and Alabama, but their presence appears to be impacting the entire continent. The dripping area looks like a large funnel, with rocks from across North America being pulled toward it horizontally…