The huge crevice at the land's edge on the coast at Bowleaze Cove in Weymouth, Dorset - once painted by John Constable - was caused after thousands of tons of earth suddenly gave way following weeks of heavy rainfall.
Water seeped into the limestone rocks and left a gap that has been measured as four foot wide and six foot deep in some places.
Now walkers and residents are being asked to mind the gap and satay well away from the unstable edge, over fears it could collapse at any moment.
The coastline, which is a UNESCO World Heritage site and a popular holiday spot for sun-seekers and fossil-hunters, is prone to landslides.
The rocky cliffs are made of a mix of permeable limestone at the top, with an impermeable clay beneath it.
I added a video to a @YouTube playlist https://t.co/jKDMPvNa12 Land slip @ Bowleaze
Following weeks of heavy rainfall of the winter and spring months, experts say the rainwater has soaked the limestone before mixing with the clay - making it far heavier than normal and causing the land to slip into the sea.
Coastguards have closed off the nearby footpath and are now warning people to stay away from the unpredictable and "unstable" area which is going to start dropping "like an elevator".
Geologist Richard Edmonds said: "There's a really big landslide in that area which has been there for years and the more rain we get the more likely it is to move.
"We have clays in the lower part of the cliff and limestone in the top, which is porous.
"It's really hard to tell how quickly that will fall away because it's driven by the weather."
"It's not a negative thing, it's all part of a progressive evolution of the coastline.
"But as it has just moved people do need to stay away. If hundreds of thousands of tonnes of rock have moved there is an increased risk of rock falls and mud.
"Mud can be quite a hazard there. It will spill out onto the beach and create a danger."
The area of Bowleaze Cove is popular with tourists and dog walkers, who are being warned to stay clear of the grassy verge entirely.
"So many people keep going up to have a look but the extra weight of all these people could cause another landslide. People need to be careful."
Weymouth resident Liz Emery noticed the crack developing over the last few weeks.
She told the Dorset Echo: "Myself or my husband walk the dogs over the fields every day.
"I first noticed a small crack and ground movement on March 21 so took a picture and a week later it had spread further across and was a lot bigger.
"I've found it fascinating, but it's scary how quickly it has happened."
Fossil hunters are also being warned to take care on the rocky beach below at Osmington Corner, as the landslip will mean more mud and clay on the shore below the landslide.
A neighbour whose home looks over the Weymouth coast said: "We've seen a fair bit of that claggy clay falling onto the beach below, so people just need to keep an eye out for any falling rocks."
This is the latest landslide to hit the Jurassic Coast in recent years.
In 2014, an expensive bungalow west of Lyme Regis in Dorset was left stranded in the middle of acres of unstable land after billions of tons of rock and mud gave way.
Last May, a huge crack similar to the one in Weymouth appeared at West Bay - where popular BBC series Broadchurch starring David Tennant and Olivia Coleman was filmed.