Hops, not grapes, put Sonoma County on the map. A century ago, the leggy bines grew in the millions. Their cones were dried for worldwide sale in oast houses or hop kilns, distinctive peaked barns with castle-like turrets that hulked like mammoths across the Russian River Valley. But by the end of World War II, the hops business had become a losing game. One by one, the county's hop kilns disappeared or fell into disrepair. Today, there are just three left in Sonoma County. This author set out to discover Sonoma's last oast houses.