This explosion is considered to be one of the two biggest non-nuclear explosions caused by man.
Heligoland had been a major centre of operations against the Royal Navy, especially during WW1 when the U boats, fast torpedo craft, and minelayers based there were an ever present threat, and constant irritation to the Royal Navy at Scapa Flow.
The island was once a British possession which was exchanged with Germany for Zanzibar in 1890.
The use of the Island as a major military base, particularly during WW1 was a serious embarrassment for the British Government.
This exercise was a genuine attempt to destroy the island's structure, so rendering it permanently unfit for further military use.

RAF Bomber Command also mounted a massive raid on 18th April '45, when 969 aircraft turned the surface of Heligoland into a moonscape, while bombing the airfield, AA guns, naval base, and U boat pens.
Over fifty german flak personnel disappeared, (presumed vapourised) during this bombardment.

Today the re-structured island is a very popular resort with families and senior citizens due to it's tax exemept status and healthy offshore, pollen free climate.
Massive Attack's latest cracking album was named after the island too, so it's got to be good.