Mostofthetime,thegroundfeelssolidbeneathourfeet.That'...
问题详情： Mostofthetime,thegroundfeelssolidbeneathourfeet.That'scomforting.Butit'salsomisleadingbecausethere'sactuallyalotgoingonunderground.Massesofland(calledplates)slip,slide,andbumpagainsteac...