National Geographic: Two decades after the worst oil spill in U.S. history, huge quantities of oil still coat Alaska’s shores with a toxic glaze, experts say. More than 21,000 gallons of crude oil remain of the 11 million gallons of crude oil that bled from the stranded tanker Exxon Valdez on the night of March 23, 1989. The oil–which has been detected as far as 450 miles (724 kilometers) away from the spill site in Prince William Sound–continues to harm wildlife and the livelihoods of local …