A floating plastic bubble, so hi-tech it is lighter than air, is attached by ropes to the walls of the National Gallery of Denmark in Copenhagen. As I step gingerly on to its see-through floor, I can peer down at the gallery 100ft below. When I'm joined by one of the museum staff, I become unsteady. We crawl around this airborne plastic yurt like babies and then, feeling giddy, stop to sit and talk about how our children might end up living in a city of such bubbles, sealed off from a contaminated earth; about who might be lucky enough to have such a refuge; how they might sing their children lullabies of a lost earth. It's an eerie conversation to have with a stranger, both of us imagining a deeply tragic future that seems highly plausible.