Covering an area of 125 hectares, this is the largest fen in lowland England. It is situated between Thetford and Diss along the border between Norfolk and Suffolk.
The fen consists of saw sedge beds and purple-moor grasslands, as well as acid fen, calcareous fen, reed fen, willow carr and alder woodland.
More than 120 rare invertebrates have been found on the site, including five which have "Red Book Data 1" status. This NNR is one of only two sites in Britain where the fen raft spider Dolomedes plantarius can be found.
Suffolk Wildlife Trust's work on the fen includes peat stripping, and habitat restoration.