Dozens of protesters will be underground in at least 12 highly sophisticated tunnels when bailiffs move in to make way for the pounds 172m runway.
News of a meeting yesterday between Randall Hibbert, the Under Sheriff of Cheshire, with police and bailiffs, sparked frenzied activity at the protesters' camps near Styal in Cheshire.
"We've moved up to amber alert," said Inverness John, one of the protesters. "We'll go up to red alert when we see them coming over the hill, and that could be any time now."
Tunnellers say that "a fair proportion" of the holes underneath the site are at least as sophisticated as the one in which their hero Swampy - aka Daniel Hooper - held out for a week in January this year under the site of the A30 at Fairmile, Devon.
Swampy has dug a more complex tunnel at Styal, but it is not clear whether he will be in it. He is at present on a bail condition which prevents him from going near the site but one protester yesterday said that no one would be surprised if he turned up. "Loads of us have got the same condition," he said.
Included in the demonstrators' armoury this time is the Cake Hole, a 50ft-deep tunnel dug over a three-month period and featuring numerous warrens, vertical climbs and 90-degree turns. It can hold at least 10 people, each of whom can be locked into large concrete blocks, and it is also understood that the tunnel features heavy doors with locks and bolts.
"They're in for a few surprises this time," said Atarra, a 16-year-old who is on the site with her mother's permission. "By the time this is all over, a lot of people will have a lot to be proud of. It will be much more difficult to remove people than it was at Fairmile."
According to Inverness John, some of the tunnellers have enough provisions to stay underground for at least six weeks.
"Whether they would like to stay down there that long is another matter," he added.
He said safety was the paramount consideration and he expressed concern that protesters had failed to get an assurance from Mr Hibbert that communications to the tunnellers would not be cut during a siege.
"We would consider that to be very dangerous," he said. "It is important that we know the condition of the people underground."
Bailiffs will also have trouble removing demonstrators from above ground. Dozens of treehouses have been built and scores of climbers are expected to chain themselves to branches.
One protester, Gary, who suffers from a spinal disorder, plans to chain himself and his wheelchair to a tree. At least 100 people were reportedly at the site yesterday and the number was rising rapidly.
Mr Hibbert was not available for comment.
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