The year that iceberg A-23A first broke away from Antarctica’s Filchner Ice Shelf, Ronald Reagan was president of the United States, and the movie “Top Gun” was setting box office records. Forty years later, the massive tabular berg—one of the largest and longest-lived bergs ever tracked by scientists—is sopping with blue meltwater and on the verge of complete disintegration as it drifts in the South Atlantic between the eastern tip of South America and South Georgia island.