The Zuidwal volcano is an extinct volcano in the Netherlands at more than 2 km below ground under the Wadden Sea, between Harlingen and Vlieland, just south west of the island Griend. The volcano was last active during the late Jurassic, and has since been covered by about 2,000 metres of sedimentary rock, most of it shale and sandstone from the Cretaceous.
The volcano was discovered in 1970 when the French oil company Elf Aquitaine was doing test drills in the Wadden Sea, hoping to find a gas field that a seismic survey had indicated. To their surprise they hit volcanic rock beneath the reservoir rock, which turned out to be an extinct volcano. The gas field went in production in 1988 and is still producing gas. Another clue indicating the presence of the volcano was the temperature. While the usual temperature at that depth is about 100 °C , they found 130 °C .
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