Stand on solid ground and look down at your feet. Go deeper - through the flesh and bones, deeper into the Earth. What's down there? It's hard to imagine, let alone visit - should you want to.

Writer and explorer Robert MacFarlane has been voyaging in this hidden world, going back in "deep time" to places measured in "millennia, epochs and aeons, instead of minutes, months and years".

Now, he has surfaced and is asking: "What will we leave behind when we are extinct?"

And he is telling us why we should care.

To MacFarlane, this image could be "an annunciation scene from Giotto".

But look more closely - in fact, it's an "avalanche of vehicles".

He abseiled down into an abandoned Welsh slate mine where locals have been dumping wrecked cars for 40 years. He says: "We are not just shaping the surface, but shaping the depth."

Will our future fossils just be "car-chives" like this, along with the inevitable strata of plastic, lethal nuclear waste, and the spines of millions of intensively farmed cows and pigs?

Or can we, as a species, start to do things better?

As a Swedish teenager, Greta Thunberg, inspires worldwide climate-breakdown protests, and Extinction Rebellion brings central London to a standstill