A "no-deal" Brexit is a curious thing, like the shark in Jaws.
Lots of people worry about it, even though nobody is quite sure what it looks like. They like to talk about it, but they fear causing a panic.
It may disappear, never to be seen again - or it might come along, and bite us. Or perhaps the shark will turn out to be a friendly, helpful dolphin.
Over the past few weeks, I have spoken to dozens of people across the spectrum of British life about how they're preparing for the possibility of the UK leaving the European Union without a deal.
These people range across big and small businesses, trade organisations, advisers, public bodies, academics and members of the public who've found themselves embroiled in planning for no-deal.
And all of them shared three things.
Firstly, confusion. The blunt, incontrovertible truth is that, with less than five months to go until the UK is due to leave the EU, nobody is sure what a no-deal Brexit would look like.
It could be chaotic or controlled, good-natured or acrimonious. We might embrace the terms of the World Trade Organisation, or come up with some other plan. Frankly, nobody really knows what's going to happen.
And that leads us to the second condition - nervousness. Most businesses get wary when they're confronted by the unknown, and a no-deal Brexit offers uncertainty by the bucketload.
That's hard to deal with, whether you're a corporate giant or, for that matter, if you're an anxious parent who worries about getting medicine for their child.
Either way, uncertainty leads to nervousness and then it turns to anxiety over the future.
And the third thing I saw, again and again, was a desire to avoid the charge of scaremongering.
The great majority of people that I spoke to said they would only have an honest conversation if it was "off the record".
So I can report the substance of what I've been told, but, in most cases, I can't tell you who it was that I spoke to.
I can tell you that every sector of British business has its own concerns about what a no-deal Brexit could mean. Will lorry drivers get the permits they need, will the energy interconnectors function on day one, will the Eurotunnel drivers have the legal right to go into each other's countries and, yes, will aeroplanes be allowed to take off from Heathrow?
Some of these fall into the category of "well, surely they'll sort THAT out". It's hard to imagine planes sitting on the tarmac at Heathrow or police helicopters being grounded - but there are people, right now, who are making contingency plans for all of these things.
But other fears fall into another category. The one marked "pressing". And most of them are allied to the fact that a no-deal Brexit would almost certainly hinder the easy movement of goods in and out of the UK.
Talk to anyone in the customs and freight industry and they'll tell you that if you add customs checks at Dover or Calais, or both, then queues will start to grow on both sides of the Channel.
And when Dover runs slowly, as it has done during periods of disruption at Calais, then a chunk of Britain's fleet of lorries ends up parked on the motorways of Kent. The distribution network slows down. The just-in-time system begins to creak.
That's the problem I heard about again and again - the one that British business is grappling with.
It is a fact that our supermarket giants are wondering how to cope with a shortage of fresh food, and even the prospect of rationing. Pharmaceutical companies are working out how to distribute medicines in the event that Dover is blocked. Car manufacturers are wondering whether they can fly in the parts they need.
The organisations who represent the nation's businesses are becoming more and more concerned that a deal might not get done.
This isn't about scaremongering, and it isn't Project Fear.
Instead, it's the reality of a swathe of British society as it gauges the potential effect of a no-deal Brexit.
It's about nerves.
You might think they're unnecessary, or you might think they're justified. But they're there.
No-deal is just a spectre at the moment - an idea that might never come to pass.
But its shadow is looming large over many people.