CALIFORNIA: Bitumen, or natural petroleum tar, has been a useful material around the world for thousands of years. In the Channel Islands, the Chumash used it as long as 8,000 years ago as an adhesive or to waterproof baskets or boats, but the islands have no terrestrial source. They could have traded for some, but scientists applied geochemical analysis and traced samples used by the Chumash back to a petroleum seep 25 miles offshore. The bitumen washed ashore in the form of “tar balls” or “tar whips.” —Samir S. Patel