When the Soviet Union had disintegrated, and Russian troops were finally leaving Latvia in 1994, much of the country’s hidden military infrastructure was suddenly exposed. Secret maps surfaced. Abandoned bases were opened. And in the middle of a vast pine forest near the Baltic Sea, one Latvian astronomer followed a rumor that had survived decades of secrecy.Juris Zhagars and his wife, Gundega, climbed into their orange Lada in Riga and drove roughly 300 kilometers to Irbene, a former Soviet military outpost that had been home to about 2,000 soldiers, scientists and intelligence personnel.They were expecting to find a giant radio…