Giants once roamed the earth. Oceans teemed with ninety-foot-long whales. Huge land animals–like truck-sized sloths and ten-ton mammoths–ate vast quantities of food, and, yes, deposited vast quantities of poop. A new study shows that these whales and outsized land mammals–as well as seabirds and migrating fish–played a vital role in keeping the planet fertile by transporting nutrients from ocean depths …