The giant annual outdoor event known as W:O:A is set to kick off Wednesday, with 85,000 headbangers due to descend on a venue of farmland and cow pastures to hear 150 bands on eight stages over four days.
Although often a muddy affair, Wacken is facing severe accessibility problems this year, prompting both promoters and police to tell anyone who has not already arrived at the showgrounds to turn back.
"The persistently difficult weather situation with rain quantities of around 40 litres (11 gallons) per square metre (yard) in the last 24 hours and the resulting condition of the campgrounds, fairgrounds and access routes means that the area could not be made ready in time," W:O:A said in a statement.
"We apologise profusely to all metalheads for the delay in providing information."
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Organisers of the sold-out festival said that weather forecasts indicated the downpour would continue, with electrical storms possible, leading them to take the precautionary measures without calling off the event entirely.
"We are making decisions hour by hour and ask for your understanding in this extremely difficult situation," they said. "You would help us most now by staying home."
It was not immediately clear how many visitors were already on site, but chaotic parking conditions were leading security staff to use tractors to move vehicles that were obstructing access.
Media reports said most of the camping areas around the fairgrounds were nearly full and the paths between them a muddy mess while roads were blocked by traffic backed up for several kilometres.
Bands including Britain's Iron Maiden, US thrash metal act Megadeth and folk punk group Dropkick Murphys are among the headliners this year.
On Wednesday, some of the ashes of Motorhead lead singer Ian "Lemmy" Kilmister, who died in 2015, were due to be brought to the W:O:A fairground as part of an "unforgettable act" in his memory.
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