The mysterious appearance of a gaping chasm in the grounds of Lewes Castle is being investigated by archaeologists.
Part of the ground at Brack Mound, a man-made hill and vantage point built in the 11th Century to help defend the town, has caved in following the wettest winter on record.
It is not yet known how deep the hole is or if it could relate to secret passages which have long been rumoured to run underneath the mound.
In the next few weeks, Sussex Archaeological Society, which owns the castle and Brack Mound, will begin exploratory work to find out how deep the hole is and whether it relates to any underground structures which have so far not been discovered.
Experts hope the torrential rain has not weakened the internal structure of the mound, a Scheduled Ancient Monument, which could mean further collapses.
Gabor Thomas, research officer for the archaeological society, said the wet weather had caused internal subsidence in the structure, causing the ground to cave in.
It is not yet known if the hole leads to further spaces within the mound Mr Thomas said: "It could be that trenches dug in the past have left voids and these have started to collapse but at the moment we don't know how deep the hole is or what it relates to.
"There have been rumours and myths surrounding Brack Mound that there are underground passages beneath it leading from the Lewes Arms to some unknown destination.
"But if that is true, it is a mystery as to why they should be there."
Mr Thomas said the hole offered archaeologists the opportunity to find out how the mound had been used in the past. In the meantime, the public are being warned to stay well away from the hole.
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