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Distance travelled ~ 709'020'800 km

Sharks and stalactites may be close to each other in the dictionary, but you would think that reality keeps them a safe distance apart. For a start, sharks aren't known for inhabiting caves, and every stalactite I've ever seen has been in a cave. Secondly, stalactites can't grow underwater and sharks can't breathe if they're taken out of water. That sounds like a clinching argument if ever I heard one, but the thing I love about science is that our world is more complicated and interesting than that. Not only did I see lots of sharks swim past lots of stalactites this week, but this weird combination tells us something fundamental about our planet. And it's not that a flock of flying sharks has started spelunking because they suddenly fancied bats for dinner.

Belize is just next to Guatemala and south of Mexico, tucked into the back of the Caribbean sea. Its coastline is littered with islands and coral reefs, but what brought Jacques Cousteau here in 1970 is circular deep blue hole in the reef. We arrived in Belize last Monday laden with SCUBA gear, all ready to explore that hole.

Going into the hole was pretty eerie. There is sand and coral right up to the edge, and then the vertical wall just drops away into the darkness. We left all the brightness and light and colourful fish behind, and sank slowly. After going down a little way, all I could see was the rock wall stretching into the gloom. I found looking away from the wall a bit disconcerting because it felt as though anything could swim out of the black, even though I knew perfectly well how unlikely that was. We kept going down further and further, and I stared at the wall, straining to see what on earth brings people here. A reef shark swam past just two metres underneath me. And then the gloom readjusted itself just in front of me and I was looking at a stalactite that was nearly a metre wide at the top where I was, and was probably 5 metres long, pointing downwards into the depths. It was monstrous. There was an overhang, like an upside-down shelf a few metres deep, and looking along it I could see other stalactites hanging down, all of a similar size. We swam along the overhang, and the sharks cruised past us a few metres further out from the wall.

Dives that deep have to be short, and we had work to do, so it was only that night that the scale and the incongruity of what I'd seen sank in.

The size of the stalactites helps you understand the size of the story they're telling. Both are gigantic, almost too big to fit into a human brain. The reason that the stalactites are down there at all is that during ice ages, sea level gets much much lower. 15,000 years ago, the last time those stalactites were growing, they were on a cliff in dry air because sea level was 120 metres lower than it is today. That's the sort of fact that you can read and understand logically, and it's something that I had known for years, but it's hard to digest properly. Read it again: 120 metres lower. That is an awful lot of ocean that wasn't there. Floating in the darkness with 40 metres of water above me, next to a rock wall that kept going downwards as far as I could see, I came closer than I ever have to really understanding the enormity of the changes that ice ages bring to Earth. Oh yeah, and there were sharks too.

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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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Distance travelled ~ 691'332'800 km

On our journey around the Sun for 23 Degrees we are focussing on three main themes that control our climate and weather, Tilt, Orbit and Spin. And at the moment we are filming the show about Spin.

The team are on the road in Ecuador. They have gone to the Ecuador rainforest to learn how solar energy powers a power circulation system in the atmosphere that dictates the climate in bands around the world. Kate is also going to drive along the equator at around 1060 miles an hour. Well not quite - her car's going 60 but the planet at the equator is spinning at 1000mph, so for a few moments she's probably the fastest person on the planet.

The next stop after the heat of South America is the Bay of Fundy in Canada. Fundy has the highest tidal range in the world and Kate and the team are going to witness it first hand.

From there it's down to Bermuda to go snorkelling to learn about our planets spin , nice work if you can get it.

One of the things I find fascinating about our planet is that it carries a record of its own history written in its rocks. Some of this history is obvious - you can't miss the impact of giant craters blasted out by asteroid strikes. But some are less obvious - unless you know where to look. Kate's studying corals as these tiny creatures hold the secret to our distant past - and how fast our planet once spun.

Every day corals lays down growth rings of limestone and these daily growth rings build up to create an annual growth ring [a bit like a tree ring]. If you count the daily rings you get 365 days a year, which is what you'd expect.

But if you look at 400 million year old corals you get a very different picture. They have rings just like the modern coral but they are a little bit narrower. But what's really surprising is that if you count the daily growth rings you don't find 365 you find 410. That means that when this coral was alive 400 million years ago the world was a very different place.

But however you measure it - in hours or days the Earths' orbit around the Sun always takes the same amount of time. A year is always constant. The only explanation for the ancient corals having 410 daily growth rings is that millions of years ago the days were shorter. So when this coral was in the oceans there were less hours in each day - in fact a day lasted just 21 hours. And for that to happen the Earth must have been spinning faster.

If we calculate back even further in time we find that around 4 billion years ago, when the Earth was still young, a day lasted just 6 hours. Which means the planet was spinning 4 times faster than today.

That's pretty wild - and we know that because of corals. Wonderful.

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Distance travelled ~ 659'976'800 km

Hurricane hunting was not supposed to be like this. The Sun was shining, there were butterflies everywhere, and there wasn't enough wind to blow out a candle on a birthday cake. On the plus side, we had to give ourselves top marks for trying and I didn't have to get drenched again.

Twenty four hours earlier, things had looked very different. We had been tracking several Atlantic storms, and finally Tropical Storm Nate was forecast to make landfall in the Gulf of Mexico as a hurricane. I've never paid that much attention to tropical storms in the past, but it turns out that storm-monitoring is surprisingly addictive. Tropical disturbances in the Atlantic often start out near the coast of Africa, and then they crawl across the ocean to the west, growing or petering out as they go. The storms move at about 15 mph, so they'd lose a race to any half-decent cyclist. That gives the nascent addict many happy days of monitoring storm strength and direction. There are also exciting milestones such as the day the storm is given its name, and most important of all, the day the maximum sustained winds first reach 74 mph and the storm is declared to have graduated to hurricane status. Most storms don't make it that far. If they drift too far north, they get broken up or run out of fuel, and they can be decapitated by high-level winds, never giving them a chance to grow.

Tropical Storm Nate was interesting because it had skipped the slog across the Atlantic ocean, and had instead formed entirely inside the Gulf of Mexico, stuck in the gap between the Yucatan and the rest of Mexico. It was pootling westwards at only 3 or 4 miles an hour, feeding off the nice warm bath it was trapped in, and forecast to hit the Mexican coastline near Veracruz as a category 2 hurricane. We thought that we finally had a winner, and off we went.

The five of us arrived in Veracruz in the dark, only 18 hours before the centre of the storm was due to hit the coastline. It was horribly hot and sticky, and the evening gloom made everything feel very ominous. The wind was picking up and we were excited and a bit nervous about what would happen in the morning.

What happened was that we learned that Tropical Storm Nate had apparently become "disorganized" overnight. I've got friends like that, but I wasn't expecting it from a giant atmospheric whirlpool. Josh Wurman (our hurricane expert) inspected the satellite images on his computer screen and made "meh" noises whenever the director asked him where exactly the storm had gone.

The tight spiral that we had seen the previous day had widened, split and was indeed looking pretty disorganized. It rained hard for a couple of hours that morning, so we did film some nasty weather, but soon the sun and the butterflies came out again. We stared at the flat calm ocean and wondered whether to blame the butterflies for flapping their wings.

In our absence, of course, the remnants of Hurricane Katia were passing over Scotland. The winds in Scotland this weekend reached twice the speeds we saw in Mexico. We are not bitter about this. Honest. We had all thought that filming a hurricane would be much easier than filming a tornado, just because hurricanes last for weeks and their tracks can now be predicted very accurately. But we learnt the hard way that the complications of our atmosphere are still not perfectly understood, and that even a large storm can vanish almost overnight if the conditions are right. But still, it's all part of experiencing the weather, and I'm actually quite glad that the town where we were was able to have a normal Monday morning, rather than dealing with the damage and flooding that a hurricane would have left behind.

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