The Ordnance Survey (OS) uses mean sea level as the starting point for measuring the absolute height of mountains, which must be at least 609.6 metres (2,000ft) above sea level. With rising sea levels there is a risk to numerous mountains which only just got classified as mountains by a few centimetres that they might get downgraded to hill.
Two mountains which are most at risk are Calf Top in the Yorkshire Dales and Thack Moor in Cumbria. Calf Top only got classified a mountain in September when the OS determined its true height is a few centimetres taller than was concluded when it was last measured in 2010, putting it just over the required height for a mountain. Meanwhile Thack Moor is only 2cm over the 609.6 metre requirement.