... and churn up big waves, too.

As Hurricane Ivan approached the U.S. Gulf Coast last September, it passed right over an array of seafloor sensors. The network detected the largest wave ever measured by instruments--one that towered more than 27 meters from trough to crest.

The 50-kilometer-wide group of 14 instruments was deployed in May 2004 to measure currents on the ocean floor, says William J. Teague, an oceanographer at the Naval Research Laboratory at Bay St. Louis, Miss. Late on the evening of Sept. 15, Ivan--moving northward at a pace of about 18 kilometers per hour and packing winds of around 200 km/hr--swept across the array over a period of several hours.

The seafloor instruments were set up to take pressure data during 8.5-minute intervals every 8 hours. As it happened, no sensors were making measurements when the eye of the hurricane was directly overhead. However, sensors did record the passing of massive waves before and after the hurricane moved through the array. During one of the data-gathering intervals, waves that often reached heights of 20 m were passing over one sensor every 10 seconds, says Teague. The largest wave in that train measured 27.7 m from peak to trough.

Computer models suggest that the storm's strongest winds--those in the wall of the hurricane's eye--could have spawned waves up to 40 m high.--S.P.