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Day 271: secrets beneath the sea

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Stephen Marsh Stephen Marsh | 18:00 UK time, Wednesday, 28 September 2011

Distance travelled ~ 696'585'600 km

One of the things that amazes me about our planet is how it carries clues to its own past. It's a bit like a giant memory stick, the trick is find the right file. And today Helen Czerski is on the trail of one of these files, but it's not buried underground it's buried deep under water.

Helen is pushing the limits of her endurance, diving 40 metres below the waves, searching for evidence of what our world looked like 20,000 years ago. It might seem odd to be going deep under the water to unearth our climate past but you'd be surprised what you find down there.

Helen's diving the Great Blue Hole 60 miles off the coast of Belize, it's as the name suggest a great big round hole that goes down over 120 metres. It was once a cave but the roof collapsed leaving the deep blue hole. It's more than just a wonderful piece of natural architecture. It's also a window into our past. Because deep down in the hole, are clues to one of the most dramatic events in our planet's history.

It's a very tricky and technical dive as it's so deep, so Helen is accompanied by a very experienced dive team. Fortunately she's an experienced and highly qualified diver herself, so she's the prefect person for the job. Though she did have to learn how to use a special facemask designed for presenters to talk underwater. Even with all the experience on show it's still a daunting dive, but she's following in some famous footsteps. Jacques Costeau explored the Blue Hole back in 1970.

As she descends Helen must carefully monitor her buoyancy, at these depths she doesn't want to go up or down too quickly, that's not good news, plus the sheer walls of the hole will make the dive feel very enclosed.

At around 40 metres she will reach what she's looking for. Here the walls of the hole are cut away and there are some incredible rock formations several metres high. These formations are stalactites, which is kind of an odd thing to find, because if I remember my geology you shouldn't find stalactites in the ocean because they can't form underwater. Stalactites are created when mineral rich water drips from the roof of a cave over hundreds or even thousands of years, leaving behind mineral deposits. Over time these build up to create the beautiful structures. But they can only form on land so what are they doing 40 metres down the blue hole?

Here's what must have happened. At some point back in time, this cave must have been above sea level, which means that when these stalactites formed, the ocean must have been much lower than it is today. These stalactites not only show us that sea levels have changed they also can show us when.

When Cousteau explored the hole they brought up a broken stalactite and when they cut a cross-section they found a series of rings, a bit like tree rings. Each of the rings represents a period of growth when the stalactite was exposed to air. That growth would stop when it became submerged again. Cousteau's stalactite shows three growth stages, so it's a record of changing sea levels over time.

Scientists have precisely dated stalactites from the Blue Hole and by comparing stalactites from around the world with other data like Antarctic ice cores, they've built up a picture of changing sea levels dating back hundreds of thousands of years. What it reveals is that sea levels here in the Caribbean and across the world, have dramatically risen and fallen over time.

Just 20,000 years ago, the sea was an incredible 120 metres lower than it is today. That means almost the entire Blue Hole cave system would have been on dry land. But the world has a finite amount of water in it at any given time. So if that huge mass of water wasn't in the oceans just where was it? Well believe it or not it was on land, but not as water but as ice.

20,000 years ago the earth was gripped by an ice age.

How did this happen, well you'll find out when our series airs...in 2012 mind you :-)

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