Hokkaido expanding radar-cloud incident

On February 19, 1958, operations personnel at Aircraft Control and Warning Site 45 on Asoiwayama, Hokkaido, were monitoring Russian aircraft when an unusual radar return developed over the Sea of Japan. The return rapidly expanded into a dense cloud-like target approximately sixty-five miles long and twenty-five miles wide while rising from about 3,000 feet to 46,000 feet. Five Russian aircraft in the area turned toward the return and proceeded toward its general location. A seismic detection described as a nuclear-explosion signal was also reported from Matsushiro during the broader incident record.