A man walking his dog behind his home fell down a 6ft-deep trench.
Ray Willis was on his regular evening walk when he fell into the ditch, which had apparently appeared overnight in Friday Woods, Colchester.
Many locals use the Army-owned woods just behind Holt Drive, Blackheath.
His wife Ann said: "He suddenly disappeared down a hole and did not know where he was.
"He went back to look in the morning and saw a trench.
"If an old person walks down there and falls, it will be dangerous."
Mr Willis was unhurt in the fall on Wednesday on land between the back gardens of houses and the woods. The Army said the land belonged to the military and was used for training.
There had been problems with fly tipping which caused dangers for soldiers.
Work had started to build an earth wall to tackle the problem.
It said signs were in place telling people the land was private but the Army recognised some locals did use it.
Plans are in place to open up some of the area for the public to use next year.
An Army representative was due to contact Mr and Mrs Willis to explain the situation.
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