The Oosterscheldekering (Eastern Scheldt storm surge barrier), between the islands Schouwen-Duiveland and Noord-Beveland, is the largest of the 13 ambitious Delta works series of dams, designed to protect a large part of the Netherlands from flooding. The construction of the Delta Works was in response to the North Sea Flood of 1953. The nine kilometre-long Oosterscheldekering (kering meaning barrier) was initially designed, and partly built, as a closed dam, but after public protest huge sluice-gate-type doors were installed in the remaining four kilometres. These doors are normally open, but can be closed under adverse weather conditions. In this way the saltwater marine life behind the dam is preserved and fishing can continue, while the land behind the dam is safe from the water. On 4 October 1986 Queen Beatrix officially opened the dam for use. At the artificial island Neeltje-Jans, at one end of the barrier, a plaque is installed with the words: "Hier gaan over het tij, de wind, de maan en wij" ("Here the tide is ruled, by the wind, the moon and us (man)"). The Oosterscheldekering was the biggest, most difficult to build and most expensive part of the Delta works. Work on the dam itself started in April 1976 and was completed in June 1986, though the road over the dam was completed only in November 1987. To facilitate the building, an artificial island Neeltje-Jans was created in the middle of the estuary. The dam is based on 65 concrete pillars with 62 steel doors, each 42 metres wide. The parts were constructed in a dry dock. The area was then flooded and a small fleet of special construction ships would lift the pillars and place them in their final position. Each pillar is between 35 and 38.75 metres high and weighs 18000 tonnes. The Oosterscheldekering is sometimes referred to as the eighth Wonder of the World, and has been declared one of the modern Seven Wonders of the World by the American Society of Civil Engineers. The dam was designed to last 200 years. Four ships were custom designed and built for this project: Mytilus, a ship equipped with various groundworking tools, such as needles to make the seabed more dense and stable. Cardium, a ship to transport and lay a special foil carpet on the seabed for the pillars to rest on. Ostrea, a ship capable of lifting a concrete pillar from the dry dock and placing it accurately on a special foil on the seabed. The ship is 85 metres long and has a portal of 50 metres high. The ship can only lift 10000 tonnes, but as a large part of the pillars is underwater and it is not necessary for the ship to be able to lift the full 18000 tonnes. This ship is considered the flagship of the construction fleet, mainly because of its larger size and power in comparison to the other ships. Macoma, a ship that works closely with the Ostrea, cleaning the foil assisting in placing the pillars accurately in their final position. The ships are named after various types of shellfish. The dam is operated by humans but if human control fails an electronic security system acts as a backup. A Dutch law regulates the conditions under which the dam is allowed to close. The water levels must be at least 3 metres above regular sea level before the doors can be completely shut. For testing, all doors are shut once in a while (each door once per month). Also emergency procedures are tested on prescheduled dates. Once the test is passed, the shutters are immediately opened again to create a minimum amount of impact on tidal movements and the local marine ecosystem. It takes approximately one hour to close one of the doors. The full dam has been closed 23 times since 1986, due to water levels exceeding or being predicted to exceed the 3 metres. The cost of operation is