Shhh, Submariners! Ever Done Zechstein Sea Ops?
The North Sea is an epeiric sea of the Atlantic Ocean. Located atop part of Europe's continental shelf, it's depth averages a mere 100 m (325 ft), with a 700 m (2300 ft) maximum. Some areas are as shallow as a scant 15 m (49 ft).
If you ever traversed the North Sea in a sub, and submarines have done so since before WW1, you can safely say you have participated in Zechstein Sea Ops. The Zechstein Sea once covered what now includes the North Sea, lowland areas of Britain and the north European plain through Germany and Poland.
Has Vigilis ever submarined the Zechstein Sea? Possibly. Submarines are always silent and strange.