The Haus des Meeres (HdM, English: House of the Sea) is a public aquarium in Vienna, Austria. It is located in Esterhazy Park in downtown Mariahilf district, one block south of the busy Mariahilfer Straße. The Haus des Meeres houses over ten thousand aquatic beings on an area of around 4,000 square meters (43,000 sq ft) inside a tall concrete flak tower built during World War II. In 2012 the Haus des Meeres attracted a record high of 436,500 visitors. The Haus des Meeres is managed by Aqua Terra Zoo, a private non-profit organization which, according to its web site, receives only marginal financial support from municipal authorities. However, the maintenance costs for the former flak tower are paid by the taxpayer.
Volunteers moved into the flak tower in November 1957. The ground floor and the bunkers were then occupied by the firefighters, which left only one and half habitable floors for the exhibition. The rest of the tower was filled with debris and open to the elements. Step by step, the aquarium expanded over the empty floors. Eventually, when the aquarium expanded to six floors, the firefighters left the buildings, and the aquarium staff was at last allowed to clear the basement of war relics.
In 1991 the building was crowned with a lightweight wraparound box with the slogan Smashed to Pieces .. In the Still of the Night (in English and German), a memorial against war and fascism designed by Lawrence Weiner. Eventually it became an obstacle to the expansion plans. The city authorities divided whether Weiner's work was a disposable billboard or a piece of artistic heritage, but by 2005 the latter viewpoint prevailed and the city picked up its maintenance costs. The old elevator, installed in 1944, operated without failure until 1997. It was replaced during an extensive renovation that enabled opening of three new floors (7, 8 and 9). The glass wings which house the reptiles and tropical birds, were added in 2000 and 2007. As of 2010, the tower houses ten habitable above-ground floors plus an open sightseeing roof deck. The tenth floor, which recreates a flak turm control vault, houses World War Two exhibits and is open only on weekends, with advance registration of visitors. The expansion had a side effect: the City of Vienna struck the "defaced" tower off the list of protected landmarks but enforced preservation of Weiner's billboard.
In 2007 the Haus des Meeres installed its largest, 300,000 liter tank for sharks. On May 7 the aquarium moved six adult blacktip reef sharks, properly sedated, to the new tank. All sharks quietly died in a few hours after awakening. Autopsy revealed fatal internal bleeding which was blamed on stress (after the accident other zoos confirmed that blacktip sharks are, indeed, prone to stress injury). The four replacement sharks of the same species that were delivered next month survived without lasting effects. The gravel lining the bottom of the tank was recycled from World War Two concrete which was torn down during the expansion. It is visible on the 55 euro cent commemorative stamp 50 Jahre "Haus des Meeres" issued by the Austrian Post in 2007.
The global financial crisis caused an abrupt drop in visitor numbers of Viennese art museums. The Haus des Meeres, on the contrary, steadily attracted more visitors every year: 258 thousand in 2007, 336 thousand in 2008, 353 thousand in 2009. In 2009 it rose to number ten in the list of Vienna's tourist attractions ranked by ticket sales, despite a significant increase in ticket price
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