The sinking city
The Popocatepetl volcano erupted for the first time 23,000 years ago
Not so long ago, you used to be able to stand on the green wooded slopes of the Popocatepetl volcano and look down on Mexico City.
Now, the view is blocked by a dense brown cloud of pollution.
Peel back this vile smog, and you uncover one of the biggest megalopolises in the world.
In just half a century, its population has increased seven-fold.