Can London work as a national park city?
A few years ago someone emailed me about turning London into something called a national park city.
Now coming from the Lake District - a national park - I was to be honest a little sceptical and very intrigued.
How could a city like London with all downsides of a successful global city - congestion, pollution and packed public transport - be classed as a national park city?
I'm sure I wasn't alone in thinking of national parks in a very purist, sacrosanct way.
But Daniel Raven-Ellison, who led the campaign to make London the world's first national park city, explained it to me.
This is about reframing and recognising what London has in terms of green space and improving on it. And there is plenty to celebrate.
London is one of the greenest cities in the world and unlike many other cities has many beautiful parks and waterways.
In terms of population density, it is not as dense as places like Paris and New York.
What this does is rebrand and re-badge the city to enshrine some green commitments.
That means London will protect its network of parks and green spaces and emphasise urban habitats are just as important as rural areas.
In a sense being a national park city it will help to promote and encourage connections with London's green space.
I get it and now the mayor has signed up to it.
But this is where it could get bumpy politically.
Sadiq Khan has targets for increasing the green canopy and planting trees but how does being a national park city sit, for example, with his decision to give the £1bn Silvertown Tunnel the go-ahead.
It will be funded by borrowing made against future revenues from a toll.
I've been down to a school in nearby Greenwich.
The pupils there are worried about more traffic including HGVs going through their neighbourhood to use the new tunnel.
Year 10 pupils say they think the mayor is being hypocritical.
How can you clean up London's air with the Ultra Low Emission Zone (ULEZ) and then build more roads?
Greens in Greenwich say all building a tunnel will do is increase traffic and pollution.
The mayor says he has to build it and ease congestion at the nearby Blackwell tunnel.
He also says it will have a bus lane that will also carry cyclists and HGVs will have to abide by stricter emission standards as the tunnel with be in the Ultra Low Emission Zone.
Being a national park city is a beautiful idea and supposedly 90% of Londoners support it but sometimes political practicalities can get in the way.
And that is the challenge.