Coastguards have warned beachgoers not to enter a “very deep hole” that has appeared on the coast.
The 12ft-deep (3.7m) formation was discovered near the Cliff Road car park in Felixstowe, Suffolk.
Sharing images of it on social media on Saturday, Felixstowe Coastguard Rescue Team warned rising tides would make the hole difficult to exit.
“As inviting as this may look during the summer, we strongly urge people not to enter,” it said.
“The team have been made aware of a very deep ‘hole’ within the River Deben estuary,” the Facebook post read.
“Although the photo does not show a true representation, it is believed to be around 12ft-deep with very steep sides.”
It added the area was continually changing with the tide and winds, and the formation would be assessed over the coming months.
