At seven metres below sea level, Holme Fen is the lowest place in Britain. It's also the largest birch woodland in lowland Europe and the most south-easterly example of a raised bog in England.

The posts were purposely driven through the soft, waterlogged peat into an underlying layer of clay in the 1850s to measure the rate the peat contracted due to marsh drainage. The idea was that as the peat sank, the posts, firmly rooted in the solid clay below, would gradually become exposed, revealing how rapidly the otherwise homogenous landscape was falling below the level of the all-too-close North Sea. Thirty years after the first post was sunk, the land had already dropped eight feet.