From Wikipedia.

On 18 April 1947 British engineers attempted to destroy the entire North Sea island of Heligoland in what became known as the "British Bang". Roughly 4000 tons of surplus World War II ammunition were placed in various locations around the island and set off. The island survived, although the extensive fortifications were destroyed. According to Willmore, the energy released was 1.31020 erg (1.31013 J), or about 3.2 kilotons of TNT equivalent. The blast is listed in the Guinness Book of World Records under largest single explosive detonation, although Minor Scale would appear to be larger.