Evidence of the forest on the land bridge to Britain

HostEvery time I look at a map of Europe, I see that narrow strip of water between England and France. It feels like this permanent gap, but we know it wasn't always there. People used to walk across it. But it wasn't just a bare strip of dirt, right? How do we actually know what it looked like down there thousands of years ago?

GuestIt’s a bit like a ghost story that fishermen have been telling for a long time. For hundreds of years, people pulling nets across the North Sea have been finding things that shouldn't be in the middle of the ocean. They would pull up heavy oak branches, or even whole tree trunks, tangled in their nets. At first, people just thought it was wood from old shipwrecks. But the wood didn't have nails or flat edges. It had bark and roots. Then they started finding bones. Huge tusks from mammoths and antlers from giant deer that were twice as big as the ones we see today. You can't have a giant deer living in the middle of a salt-water sea. They need grass, they need cover, and they need a lot of food.

HostBut couldn't those things have just been washed out there by big floods? A river could carry a tree a long way. That doesn't mean a forest was growing right there on the sea floor.

GuestThat was the big question for a long time. But the proof is in the dirt itself. When people started looking at the actual floor of the sea, they found layers of something called peat. If you have ever been to a bog or a marsh, you know that thick, black, spongy soil. It's made of plants that died but didn't fully rot because they were underwater in fresh pools. We found huge slabs of this peat on the sea bed. And inside that peat, the trees are still there. We have found stumps of oak and pine trees that are still standing upright. Their roots are still tucked into the soil they grew in thousands of years ago. They weren't washed there. They grew there, died there, and then the sea rose up and swallowed them whole.

HostWait, if they're just sitting there in the water, why haven't they rotted away by now? Wood usually disappears pretty fast once it gets wet and stays that way.

GuestIt's all about the air. Or rather, the lack of it. When those forests were flooded, they were quickly covered by a thick layer of cold, wet mud and fine sand. That mud acts like a heavy blanket. It keeps out the oxygen that the tiny bugs and fungi need to eat the wood. It's almost like they were vacuum-sealed. When we bring that wood up today, it's often so fresh that you can still smell the pine or see the rings in the oak as if it was cut down last year. Because no air can get to it, the wood stays solid instead of turning into mush.

HostSo we have the stumps and the big branches. But that just tells us there were trees. How do we know it was a deep, thick forest and not just a few scattered groves here and there?

GuestThat's where we have to look at the tiny things. We use these long metal tubes to poke deep into the sea floor and pull out a long pipe of mud. It's like taking a sample of a layered cake to see what's inside. Inside that mud, we find millions and millions of tiny grains of pollen. Pollen is the dust that flowers and trees make to spread their seeds. It's very tough and hard to destroy if it stays in the right kind of mud. Every tree has its own special shape of pollen. When we look at those grains under a glass, we can see the whole history of the land. We see the shift from grasses to birch trees, and then to the big, heavy woods of oak and elm. The sheer amount of pollen tells us it wasn't just a few trees. It was a sea of green that stretched for hundreds of miles.

HostIt's wild to think about. This huge, hidden world just sitting under the boats. But the water must have come in at some point. Was it a slow change, or did it all happen at once?

GuestIt was likely a bit of both. For a long time, the ice was melting and the sea was creeping up, maybe an inch or two a year. The people living there would've seen the marshes getting bigger and the woods getting smaller over their whole lives. But there was also one massive event. A huge landslide under the water near Norway caused a giant wave to race across the North Sea. That wave would've been taller than a house. It would've ripped through those forests, tearing up the trees and drowning the low lands in minutes. We find the evidence of that too, a layer of sandy rubble mixed in with the forest peat, showing where the sea finally broke through and stayed.

HostSo the forest was basically wiped out by this wall of water. It makes those old stumps feel a lot more like a crime scene than just some old wood.

GuestIt really does. We even find the things people left behind in those woods. We have found flint tools and fishing spears made of bone stuck in the same peat as the tree roots. These people weren't just passing through. They were hunting in those woods, gathering nuts, and making homes. They were part of that forest world right up until the end. We're still finding the tracks of their children and their dogs pressed into the mud flats that are now buried under forty feet of salt water.

HostThose footprints in the muck really change the map, turning a cold stretch of sea back into the woods where people once walked.