A stunning photo went viral over the weekend, revealing a supertide that turned an 11th-century French abbey that is usually surrounded by sheep into an island swallowed by the sea. The image, from AP, is shown here.
Similar photos from other news agencies also ran rampant on the Web. Thousands of people arrived on the coast of Normandy to watch the spectacular 14-meter-high surge of water envelop Mont Saint-Michel, the enclave around the abbey—usually accessible only by a causeway, which was overtopped by the tide.
The articles that accompanied such photos failed to explain why this supertide happened, or made vague references to the sun, or the moon, or the alignment of the sun and moon, or the eclipse that occurred the same day—and without attribution to any expert or scientific institution. Many also referred to the March 20-21 event as the “tide of the century,” even though it arises every 18 years. The previous occurrence was in March 1997, and the next one will be in March 2033.
So what caused the supertide? A strong clue is the repeat of “March” in the dates. Tides are driven by the moon’s gravitational pull. The sun’s gravity has an effect, too, although much smaller. When the sun and moon are aligned with Earth, their combined pull is greatest, creating what are called spring tides. They occur twice every month, but are often greatest in March and September, during the spring and autumn equinoxes.*
This year the moon and sun aligned very exactly, causing Saturday’s eclipse. “If we see a solar eclipse there will be a spring tide,” Hal Needham, a climatologist at Louisiana State University, told me. “But if we see a spring tide it doesn't mean we will see a solar eclipse.”

But what explains the recurrence of March for the highest of tides?* And why the 18-year cycle? This gets trickier. The best explanation, which Needham emailed me, comes from the U.K. Met Office, Britain’s official meteorological agency. Experts at the Met wrote in a blog that some spring tides are higher than others “because tidal forces are strengthened if the moon is closest to earth in its elliptical orbit. Tide forces are also enhanced when the sun and the moon are directly over the equator. For the sun this happens on or around 21 March or September. The moon’s orbit also takes it above and below the equator over a period of 27.2 days. Just as with the sun, the tide-generating forces are at their greatest when the moon is directly overhead at the equator.”

The Met also noted, “Very large spring tides occur when these astronomical factors coincide. Approximately every 4.5 years the moon is closest to the earth, and is also overhead at the equator.” So when the moon and sun align, and they are directly over the equator, and the moon is closest to earth, we get the so-called supertide.
Needham also noted, however, that the weather can have a greater influence on tides than any of these factors—witness Hurricane Sandy in New York City or Hurricane Katrina in New Orleans. One 18-year supertide might be higher than another depending on whether winds are pushing water up against a coastline and whether atmospheric pressure is lower than normal, which can cause a local rise in sea level.
This weekend’s supertide was felt around the world and was most extreme in places that are prone to excessive tides. In North America, the most famous site is the Bay of Fundy in Nova Scotia, where the tide can rise and fall by 10 meters even on a ho-hum day. There is no thousand-year-old abbey in the middle of the bay, however, so I guess people were not posting photos from there on Saturday.

*Clarification (3/26/15): This paragraph was edited after posting to describe more accurately the frequency of spring tides.