The Maeslantkering is a massive storm barrier located on the Nieuwe Waterweg, a waterway in the Netherlands. It is designed to protect the region from flooding during severe storms. According to the most commonly cited count, it has closed to the sea only twice in real storms: on November 8, 2007, and December 21, 2023. The barrier is an engineering marvel, featuring two floating doors, each 210 meters wide, 22 meters high, and 15 meters deep. These doors are mounted on 237-meter-long arms, each weighing 6,800 tonnes, and are connected to a massive hinge measuring 10 meters in diameter and weighing 680 tonnes. This hinge is the largest of its kind and was specially designed for the barrier. The operation of the barrier is both precise and complex. The doors are normally kept empty and floating in dry basins. About 30 minutes before a closure, the basins are flooded, and then motors turn the doors toward the center of the channel. As they fill with water, the doors descend to the bottom threshold in about two hours. Normally, the waterway allows a free passage of 360 meters for ships. The barrier was constructed between 1991 and 1997 at a cost of 450 million euros and was inaugurated by Queen Beatrix on May 10, 1997. The doors are slightly taller than the Montparnasse Tower in Paris. The decision to close the barrier is based on precise calculations rather than subjective impressions. A computer connected to weather data automatically commands the closure when a surge of 3 meters is expected. The official threshold for closure is 3 meters above the NAP (the Dutch reference zero) in Rotterdam and 2.9 meters in Dordrecht. About 300 ships pass each day in front of the barrier, and a closure can cost the port around 10 million euros per day. This is why the decision to close is based on calculations rather than impressions. Every September, Rijkswaterstaat conducts a general rehearsal of the barrier's closure with the Hartelkering, its twin barrier, to ensure the system is ready for the storm season, which runs from October 1 to April 15. In 2026, the rehearsal took place on September 12, with navigation suspended for several hours. In addition to the annual test, a full closure is conducted every seven years. The barrier protects about 2 million people in Rotterdam, Dordrecht, and surrounding areas. With the rise in sea levels, projections suggest that the barrier may need to close about once every five years over the next fifty years.