Norfolk Wildlife Trust’s Cranberry Rough nature reserve has a hidden secret, for lying beneath rough grassland, fen, and scrubby woodland is a lost lake, or ‘mere’ to use the exact term.

Cranberry Rough’s Hockham Mere is situated in the Brecklands, a unique UK landscape which contains twelve active meres in total.

These naturally occurring spring fed bodies of water are incredibly rare and known as fluctuating meres.

They can, rather oddly, contain more water during a hot summer than in winter.

This is due to a delayed response to groundwater levels, with the quantity of rainfall from previous years dictating the amount of water that percolates up through the porous chalk.

These exceptional geological features have remained intact since the end of the last Ice Age.

The largest, at seven hectares, is Fowl Mere, but the now vanished Hockham Mere is estimated to have been a whopping eighty-one hectares, with core samples suggesting it may have been up to thirty feet deep.

Like the other Breckland meres, it formed over 10,000 years ago, probably by the dissolution and collapse of the underlying chalk bedrock.

Boreholes have shown that the lowest layer of sediment in Hockham Mere is sand, which would have been washed or blown in from the bare land left behind as the last glaciers retreated.

Since then, the basin has filled with thick silt, and a........

© Norwich Evening News