Meteorologists have long recorded the weather at land-based stations. But nearly three-quarters of the planet is covered by water, and that’s where most weather takes place. Thousands of ships have criss-crossed the oceans, noting the weather in handwritten logbooks that for decades sat forgotten in bookshelves and basements. In a sometimes-obsessive quest, thousands of Old Weather volunteers have extracted millions of observations about barometric pressure, wind speed, air temperature and ice from the old logbooks. These are fed into a huge dataset at the U.S. National Oceanic and Atmospheric Administration, creating what NOAA calls “a dauntingly complex, high-resolution, four-dimensional reconstruction of the global climate that estimates what the weather was for every day back to 1836.”