The fens are mainly in the counties of Cambridgeshire and Lincolnshire and partly in Norfolk and Suffolk in England which is part of the United Kingdom.
There is a map of the fens available in the Fens App.
The fens covers an area of approximately 400,000 hectares or 1,000,000 acres.
This is equivalent to:-
over 1,500 square miles or
5,000 football pitches or
400 square kilometres.
Here’s a link to some maps to help you explore:-
