An instrumented buoy just recorded a wave almost 24 meters (78 feet) high in the Southern Ocean some 700 km south of New Zealand. This was the largest wave ever definitively measured in the Southern Hemisphere. As with most such waves, it was formed in a storm, where the complex mathematics of wave science show the occasional monster arises when the force of several waves combines into one. The sailors' saying that every seventh wave is a giant hasn't been borne out by science, but there's a titanic amount of energy in play. By the way, the popular term "rogue wave" doesn't have a precise definition: according to NOAA a rogue wave is any wave that's "large, unexpected, and dangerous."
Swordboat captain Linda Greenlaw once wrote that "What's the biggest wave you've ever seen?" is a dumb question because captains are a little busy during big-wave conditions: they just divide sea states into "'This sucks" and "This really sucks." Nevertheless, the biggest wave on record was measured with surprising calmness and precision. This king of waves was spotted in 1933, when officers on the bridge of the oiler USS Ramapo triangulated a wave at 112 feet (34.1m) from trough to crest during a severe Pacific storm. This wasn’t a wild guess: as laid out carefully in the captain’s account, it involved a bridge officer sighting a horizon-filling wave dead astern and lining its top up with a boom on the mainmast. It sounds clinical, but it must have been terrifying even to a 477-foot ship. (Given the conditions, the measurement was still not "exactly exact," and the wave may have been a few feet shorter - or taller.)
A 90-foot-ish wave was photographed crashing over the deck of the supertanker Esso Languedoc in 1980. The wave that hit the RMS Queen Elizabeth II during a hurricane in the Atlantic in 1995 was estimated at 95 feet (29m). In 2000, the British research ship RRS Discovery measured a wave of the same height in the Atlantic west of Scotland. A few years later, the RMS Queen Mary took a hit from a rogue wave estimated at 92 feet high. In 2004, Hurricane Ivan fostered a wave in the Gulf of Mexico that, according to pressure sensors deployed by the U.S. Naval Research Laboratory, was in the 90-foot range.
Many huge waves are seen but unmeasured: famed explorer Ernest Shackelton, in a small vessel in the Southern Ocean in 1916, saw a wave so large "I called to the other men that the sky was clearing, and then a moment later I realized that what I had seen was not a rift in the clouds but the white crest of an enormous wave. During twenty-six years' experience of the ocean in all its moods I had not encountered a wave so gigantic. It was a mighty upheaval of the ocean, a thing quite apart from the big white-capped seas that had been our tireless enemies for many days. I shouted, 'For God's sake, hold on! It's got us.'" They survived, barely.
Waves that strike a seaside cliff or lighthouse will wash much higher than the wave's actual height, making the wave size a guess, but it must have been an extraordinary wave that hit the Eagle Island lighthouse off Ireland in 1861. According to contemporary claims, the wave smashed windows 66m (220ft) above the normal sea level, 40m of which was a cliff and the rest the lighthouse itself. A wave hitting the Fastnet Lighthouse in the same region in 1985 splashed 47m (154ft) above sea level.
Waves not born of storms can become huge, too, mainly in certain areas of the world where bottom topography, shorelines, and/or reefs can concentrate incoming water. One such place is Nazaré in Portugal, where last November a Brazilian surfer named Rodrigo Koxa rode a 24.4 meter (80-foot) wave. There are requirements for photography and documentation for official records, and Koxa's wave is the largest ever confirmed as such a record even though there are online videos claiming to be of larger conquests. Surfing legend Laird Hamilton rode a wave at Cortes Bank in the Pacific he told author Susan Casey (see her book, The Wave) looked to be not only past the 100-foot mark but closer to 120 (36.5m), but there were few witnesses and no photographs.
Fortunately for mariners, the 200-foot (over 60 m!) waves described in the Preston-Childs thriller The Ice Limit don't exist, despite those authors' normally praiseworthy efforts to put in correct scientific detail. It's hard to imagine they could, given the sheer mass of water that would have to pile up and the force or gravity tearing it down. But we used to think 112-footers didn't exist, either.
(The Ramapo account is from Lt Cmdr R.P. Whitemarsh, Proceedings, August 1934. Reprinted in Gardner Soule, Under the Sea, Meredith Press, NY 1968)