Her eis some info found on Wikipedia:
Heligoland - Wikipedia, the free encyclopedia
During World War II the islanders remained on the main island. The first bomb dropped on German soil during the war landed here on 3 December 1939 by mistake from a British bomber. There was also a large allied air raid on the island on 15 October 1944, destroying many of the buildings of the Unterland. Then, on 18 April, 1945, over a thousand Allied bombers attacked the islands, leaving nothing standing. The civilian population was protected in rock shelters, most of the 128 people killed being anti-aircraft crews. The islands were evacuated the following night.
From 1945 to 1952 the uninhabited islands were used as a bombing range. On 18 April 1947, the Royal Navy detonated 6,800 tonnes of explosives ("Big Bang" or "British Bang"), creating the biggest non-nuclear single detonation in history. While aiming at the fortifications, the island's total destruction would have been accepted. The blow shook the main island several miles down to its base, changing its shape (the Mittelland was created).
In 1952 the islands were restored to the German authorities, who had to clear a huge amount of undetonated ammunition, landscape the main island, and rebuild the houses before it could be resettled.
Heligoland is now a holiday resort and enjoys a tax-exempt status, as it is part of the EU but excluded from the EU VAT area and customs union, and consequently, much of the economy is founded on sales of cigarettes, alcoholic beverages and perfumes to tourists who visit the islands.