<p><strong>Floods in <a href="http://travel.nationalgeographic.com/travel/city-guides/venice-italy/">Venice</a> usually mean breaking out the waders, but a few adventurous souls in St. Mark's Square threw in the towel Sunday and stripped down to swimsuits. </strong></p><p>It's normal for <a href="http://travel.nationalgeographic.com/travel/countries/italy-guide/">Italy</a>'s "eternal city" to flood in winter, but Sunday's 5-foot (1.5-meter) inundation is the sixth highest since record-keeping began in 1872, according to the Reuters news service. (Interactive map: See how much of <a href="http://ngm.nationalgeographic.com/2009/08/venice/venice-animation">Venice floods at different water heights</a>.)</p><p>"Usually flooding occurs when there is high tides and rain." Recently "strong winds from the south further enhanced the high-tide conditions," explained <a href="http://www.rsmas.miami.edu/users/swdowinski/bio.html">Shimon Wdowinski</a>, a geophysicist at the University of Miami.</p><p>Most parts of Venice don't flood during the <em>acqua alta </em>("high water"), but on Monday three-quarters of the city was underwater. As of Tuesday, the water level has begun to recede, but it may take a while, Wdowinski said. "Venice sits in a lagoon that has three outlets to the Adriatic Sea. In cases of heavy rain, like now, it takes time for the water to flow out."</p><p>A barrier to protect the city from repeated winter flooding—known as the <a href="http://www.salve.it/uk/soluzioni/acque/f_mose.htm">MOSE project</a> —is due to be finished by 2014. Until then, "there's not much that can be done," Wdowinski said. (See <a href="http://news.nationalgeographic.com/news/travelnews/2012/01/120112-venice-flooding-travel-science/">"Injections Could Lift Venice 12 Inches, Study Says."</a>)</p><p><em>—Ker Than</em></p>