Short answers, pulled from the story.
The North Sea covers an area of 750,000 km2 and has a volume of 54,000 km3. Its mean depth is 90 m, though the Norwegian Trench reaches a maximum depth of 725 m and the Devil's Hole, located 320 km east of Dundee, Scotland, descends to 230 m.
Seven countries share the North Sea coastline: Great Britain, Norway, Denmark, Germany, the Netherlands, Belgium, and France. Around 185 million people live in the catchment area of the rivers that discharge into the sea.
Onshore oil was discovered around the North Sea as early as 1859, and natural gas as early as 1910. The major offshore discovery came in 1969, when Phillips Petroleum Company found the Ekofisk oil field, with commercial exploitation beginning in 1971.
A storm tide in 1228 is recorded to have killed more than 100,000 people. The Second Marcellus Flood of 1362, also known as the Grote Manndrenke, claimed more than 100,000 lives and permanently destroyed large sections of coast, including the lost city of Rungholt. The North Sea flood of 1953 cost more than 2,000 lives across several nations.
The destruction of the offshore oil platform Piper Alpha in 1988 killed 167 people, making it the largest single humanitarian catastrophe in the history of North Sea oil production.
The North Sea became the main theatre for surface naval action in World War One. Britain's Grand Fleet and Germany's Kaiserliche Marine faced each other there, and a British blockade maintained partly through the North Sea Mine Barrage restricted the Central Powers' access to crucial resources. Major battles included the Battle of Heligoland Bight, the Battle of the Dogger Bank, and the Battle of Jutland.