- This frozen tundra may look barren, but for me as a paleobotanist, this is geologic heaven.

(dramatic music) There are a few small arctic wildflowers and some scrappy ground cover, but the nearest living tree is 1000 miles south of here.

Finally, we reach what I've been looking for.

Wow.

Oh my God, there's some amazing stumps here.

Look at this.

Sticking outta the hillside are a bunch of weird brown rocks.

These are petrified tree stumps.

Geologists have dated these trees to around 50 million years ago.

Back then, this treeless hillside would've been completely different.

(gentle music) There's a lot of clues here to how this forest got to be the way it is.

The first is that all these stone trunks are surrounded by coal, black rock, which is a very strong signal that these trees, when they were growing, were growing in a swamp.

The second clue are these radiating roots.

In swamps, roots grow out and not down.

(gentle music) But what kind of trees were these?

(hammer tapping) These rocks are just chunky silt stones that are quite near the petrified forest, and I'm breaking them apart because I'm looking for fossil leaves.

If I can find the leaves that go with those trees, I can put that forest back together again.

Come to Papa.

(gentle music continues) (hammer tapping) Ah, now here we go.

Here's a closeup of that thing.

- What have you got?

- This is Metasequoia.

Little needles.

It's the dawn redwood.

This is conifer, and these whole sprigs will fall off as they've done here and land in the mud.

This used to be mud.

Now it's a rock that would go with those trunks at the petrified forest.

(gentle music) Together, these leaves and tree trunks give us a window into the past.

50 million years ago, this dried barren tundra was a warm, humid swamp, (water trickling) (insects chirping) covered with lotus plants, ferns, and metasequoia trees that were as tall as 150 feet.

(gentle music continues) (water trickling) The area covered in ice today was once a massive forest that stretched all the way up to the edge of the Arctic Ocean.

Water ran off the land, forming a surface layer of fresh water.

Instead of sea ice, the North Pole was covered in subtropical floating ferns.

(gentle music)