A huge hole - at least three feet deep - has opened up in the ‘River’ ride used by horse riders below Church Hill at the Ashdown Forest.
This has been caused by recent rains.
Forest conservators said it is extremely dangerous so riders are advised to avoid it.
It will take a lengthy dry spell to create the right ground conditions to make a repair and once the weather has stabilised the work will take a considerable amount of time.
Conservators say there are two alternative crossings so riders should use them until further notice.