A single coral core off West Maui has handed researchers a 250-year environmental diary, useful news for anyone trying to understand what has damaged Hawaii's reefs.

• A massive coral colony off West Maui yielded a core covering roughly 1760 to 2013.
• University of Hawaii at Manoa researchers read the chemistry inside it to track how the water around Olowalu Reef changed over time.
• The sharpest shifts show up in the 20th century, when sugar cane farming spread across West Maui.
• That points to farm runoff, not just warming seas, as a long-running stress on the reef.

Outlook: The record gives a baseline for judging how much Olowalu Reef can recover now that the plantations are gone.