The biggest, baddest waves aren’t born that way. Winds at sea generate waves that average ten feet high; during storms, 30-footers are common. But what creates waves the size of office buildings, including the ones big-wave surfers covet and coastal dwellers fear? In a word, land. A wave approaching a shoreline meets shallower and shallower water, slowing the wave’s leading edge. Now much of the energy that had been propelling the wave forward has nowhere to go but up, so the wave grows taller. Unlike the waves we enjoy at the beach, tsunami waves don’t break because they don’t get steep enough. Energy distributed throughout the water column and wavelengths extending a hundred miles give them frightening stability. They arrive as towering, surging masses.

25 feet

Teahupo’o, Tahiti’s waves are modest in height but surfers call the thick lips the world’s “heaviest.”

29 feet

As the tide comes in on Hangzhou, China, a wave called the Silver Dragon travels up the Qiantang River, opposite the direction of the river’s flow. This tidal bore is largest in September.

30 feet

The Banzai Pipeline in Oahu, Hawaii, gets our vote for the most dangerous surf wave. It tosses boarders directly into a shallow reef. At least ten people are believed to have died there.

50 feet

The Indian Ocean tsunami ten years ago traveled at speeds reaching 500 miles per hour and barged up to a mile inland. It killed some 200,000 people, making it the deadliest wave known.

78 feet

Garrett McNamara holds the record for the largest wave ever surfed, set in 2011 in Nazare, Portugal. Last year he claimed to have surfed a 100-footer also at Nazare, but the height hasn’t been confirmed.

84 feet

Until 1995, most scientists dismissed sudden, unexpected swells known as rogue waves as maritime myth. But on New Year’s Day of that year, a monitoring platform off Norway’s coast recorded a single 84-foot wave surrounded by 20-footers. The simplest explanation for these monsters is that two or more waves meet and align in such a way that their crests combine into one much larger crest.

100 feet

An earthquake followed by a landslide in 1958 in Alaska’s Lituya Bay generated a wave 100 feet high, the tallest tsunami ever documented. When the wave ran ashore, it snapped trees 1,700 feet upslope. Five deaths were recorded, but property damage was minimal because there were few cities or towns nearby.