If you happen to be passing through Hatton Cross this weekend, you will see a swollen army of police officers equipped with weapons and video cameras and peeved expressions.
They will greet you at the entrance to the Tube stations, to the airport, and on every corner, and they will probably film your face as you walk by. They are ready and raring to use the new anti-terror laws. So you might wonder - has Osama bin Laden been spotted in the vicinity?
No. A legion of environmentalists, committed to non-violent direct action, have erected an array of marquees and wind turbines and compost toilets in an empty field. As I spent this week with them, I discovered they have one purpose: to urge us to listen to the world's scientists and cut back on our greenhouse gas emissions - before we descend into climate chaos we cannot reverse and may not survive.
Alice James is sitting outside the bright white Children's Tent in the makeshift protest-city that has risen in an empty field next to Heathrow. The 26-year-old is a PhD student in atmospheric physics and she is watching her son bounce merrily on a trampoline as she explains, "We are trying to say to the people over there" - she points at Heathrow - "Do you know the connection between your flight and the hurricanes and the floods and the droughts we are seeing intensify across the world? Do you care?" She is drowned out by the roar of a cheap flight far above. Sitting later in my leaking tent, watching the Climate Camp bustle by, it seems like a surreal splicing of Glastonbury, a science seminar, and the civil rights movement. On every corner, people are discussing the nature of the warming world we are rapidly bringing to boiling point.
At one end, Mayer Hillman, the 76-year-old climate-change campaigner, is saying to a crowd: "We are on a trajectory towards the extinction of life on earth. In the main, people have done this unwittingly, so it can be excused. But now we know what we are doing, and it cannot be excused."
Further along, hundreds more are discussing how Britain can claw back its emissions, whether it's through a new, much better coach network or a Europe-wide electrical super-grid. These "unemployed layabouts" and "stupid hippies" (copyright Talksport Radio) must be the most scientifically qualified protesters in history, with every other person seemingly a science graduate.
I recognise an undercover journalist from a right-wing newspaper. "This is terrible!" he says "I've been sent to find stories about drug-addicted layabouts and they're all nice people with PhDs."
An impromptu barbershop quartet dressed as air stewards has formed. "Your exits are here, here, here and here," sings one. "Unfortunately, there are no exits from the planet." The next day, this is reported in the right-wing press with the headline: "Protesters dress as pilots to raid airports."
The contrast between the actual camp in here and the media camp Out There - the one ferverishly imagined by a press that is shut out - is often this bizarre. Ben Healey from the camp's media team tells me: "The press has been fair on the whole but unfortunately it has been infiltrated by a militant fringe led by the Evening Standard and some unsavoury elements have piled in behind them." I keep hearing on the radio about "militants flooding in" to the camp, and try to figure out who they are exactly. The hippies who have brought big bunny rabbits and chickens along? The big guy with the shaved head who starts quoting Gandhi at me?
Perhaps 83-year-old Ethel Bull is The Militant. Leaning on her walking stick, she says to me, "I'm going to be made homeless [if they build a third runway] and I want to know why. Where do you go when you're 83?"
She is one of the legion of locals from the surrounding villages of Sipson, Harlington and Harmondsworth who have embraced the camp as one of the last ways to save their homes. Derby Bahia, a mechanic, enthuses: "It's fantastic. I've never seen anything so wonderful in my life. The only thing I'm worried about is the police. Why don't they go and find some murderers instead?"
Linda McCutcheon, in her 60s, looks out across the village and says, "If this runway goes ahead, 41 years of my life will be under concrete. My children were born in that house there. I live there. My first family home is just beyond there. All gone."
Small teams of protesters - "affinity groups" - are already spreading out from the camp to protest at the Department for Transport and to block the private jets of the super-rich at Biggin Hill and Northolt private airports. But on Wednesday, we all gather in the biggest marquee to decide on the target for the main demonstration this weekend.
All the decisions here are made by consensus: we decide them collectively and carry them out collectively. There are a slew of deserving targets submitted for discussion: the offices of Heathrow's operator, BAA, which has tried to have this protest declared illegal; the building for the new ecocidal disaster of Terminal Five; the garden of Sir Clive Solely, the former Labour MP who became campaign director of Heathrow Future; a carbon offset company to punish them for the keep-on-flying myths they peddle, and more.
Everybody takes the decision with thoughtful seriousness, offering complex and media-savvy reasons for each one that we then break into smaller groups to discuss. From speaker to speaker, there is a plain commitment to never use violence against people: as protester Richard George puts it, "The only thing we are armed with is the peer-reviewed science."
There seems to be a general agreement, too, that The Enemy here is BAA, the airline industry and the Government's current policies. Time and again, speakers stress they don't want to target passengers. A few people cavil at that. One says: "If you're sitting in a drought in Africa caused by global warming, you'd find it a bit odd that we don't want to even delay Mrs Jones by a few hours to make our point on their behalf." Most people shake their heads and say disrupting passengers will play into the hands of the camp's enemies.
Scattered across the meetings that follow, the camp seems confident enough in its shared goals to express a few disagreements.
There is a division between people who believe the solutions have to come largely from governments imposing carbon rationing and investing in large renewable infrastructure projects, and more anarchist-minded protesters who think this is authoritarian and makes the protesters too complicit in existing power structures.
Bemused at being attacked as pro-government, Mayer Hillman says to one anarchist: "You go down to Heathrow airport and tell them you want to build an anarchist grassroots society from the bottom up, now will you please give up your flight? They'll tell you to fuck off."
Everyone agrees, though, that we must not use violence against human beings. Yet the police seem determined to use anti-terror laws sold to the public with the promise they would only be used against wannabe suicide bombers. On Tuesday night, a battalion of 40 officers turned up at the camp with full riot gear - and a cavalcade of ambulances - demanding entry. The protesters stood in front of them in the rain, folded their arms, and chanted: "Out! Out!" After hours of staring angrily, the police finally shambled out.
Not every act of police over-reaction has been rebuffed so successfully. When a group of the campers went out to join up with a march by local residents, the police swooped and surrounded them, forcing them on to a bizarre march to Heathrow and back sandwiched between police vans. Katie Smith, 25, who works as a home help for the elderly, had a banner reading "Camp for Climate Action" seized. When she asked what was illegal about it, the officer snapped, "You could do anything with a banner like that." She asked where she could get it back, and they said she could go to the police station in Heathrow airport - which the protesters are banned from entering. As I gape at this, I have to shake myself and remember: we are the ones on the side of almost all the world's scientists, and we are only exercising our democratic right to protest.
When I see the police seizing bags and thrusting cameras into the faces of these peaceful people, I keep comparing it in my mind to the policing of fuel protests back in 2000. A group of truckers, enraged that their God-given right to burn cheap oil was being infringed by mildly green taxes, brought Britain to a standstill. Supermarket shelves emptied; people began to panic. Through it all, the police did nothing, treating the barricaders like mildly naughty children. The very newspapers now damning direct action as "undemocratic" and "disgusting" cheered them on. So according to the police and the right-wing press, protesting to speed up global warming is fine, even if it causes food shortages; but protest to halt global warming and you become a mini Bin Laden.
Standing not far from these police vans, environmental campaigner George Monbiot summarises the stakes to a pensive crowd. He quotes from a scientific paper by Nasa's Professor James Hansen, which says that the last time the world warmed by 2-3 degrees C in such a short time, the world's major ice sheets collapsed very quickly - and sea levels rose by 25 metres. "If that happens again," he says, "it would inundate the areas where 60 per cent of human beings live." The assembled Climate Camp listens to this statistic with a sad but unsurprised revulsion.
By gathering here, we have shown that at least a few thousand people are sane enough to wave and shout as the ice-sheets fall - even if the rest of the world strolls silently by into a shiny new jetplane to Hell.