…while still remaining on the land surface?
In the British Isles the answer is 3 metres (about 10 feet) below sea level.
And the location, North Slob, County Wexford, Ireland, is alarmingly close to the sea.
In the 1840s, a sea wall was built around about 1000 hectares of mudflats and a pump was installed, so that the land could be dried out and used for farming. As the water was pumped out, the ground compacted and the surface dropped below sea level.
I haven’t been to the North Slob, though there is access at the Wexford Wildfowl Reserve. However, I have been to the lowest spot in the UK, where the situation is very similar and nicely illustrated.
This is Holme Fen – also a nature reserve, and here you are at 2¾ metres (about 9 ft) below sea level. (The sea is a bit further away too!)
In the 1840s this spot was about ½ mile from an area of shallow open water known as Whittlesey Mere.
When plans were proposed to drain the mere, and claim its land for farmland, local landowner, William Wells, realised that the spongy peat would shrink as the water was removed, and decided to install a gauge to measure the change.
In 1848, choosing what was already one of the lowest points in the country, he arranged for a long vertical pole to be driven into the ground, right through the peat to the clay layer beneath. To do this the pole had to be about 7 metres long. Once it was in place the top was cut off level with the ground surface.
Then the mere was drained – the water was pumped away, the ground began to shrink, and the pole began to reappear.
In 1851 the original wooden post was replaced with an iron one, set up with its top at exactly the same height, where the ground surface used to be. This is what you can see today.
By 1957, so much of the post had reappeared from the peat that it had become unstable and steel guys were added to support it. At the same time a second post was added and on this one the level of the land at different dates is indicated.
We’ll find out more about this area next week.