Coastguards are urging walkers and fossil hunters to avoid a two-mile stretch of the Dorset coast, where a landslip has happened.
The landslip has left sections of cliffs between Charmouth and Lyme Regis "dangerously unstable and liable to subsidence without warning".
It can become like walking in black glue, Portland Coastguard warned. The area is known as the "Black Ven".
People should not walk there during "high water" due to the mud pools.
A Portland Coastguard spokesman said: "Avoid the area at high water, do not walk or climb over it.
"There can be a fresh fall without warning as well as the landslip, the area is notorious for mud pools
"There is a danger of people trying to climb over it and getting stuck, which can be quite frightening.
"That particular section of the coastline can move at any time.
"Whether walking along the beach or cliff top, ensure that dogs are leashed and children supervised at all times."