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The second instalment of the series follows the Earth's journey from the start of January to the Spring Equinox in March. Available on iplayer. What did you think?

Kate begins the film on a day with a very significant point in our Earth's journey - Perihelion. Kate climbs Aonach Mor mountain, one of the highest mountains in Scotland, which brings her as close to the Sun as she'll ever be for the entire year.

This however is not because of where she is but because of the point the Earth has reached in its orbit around the Sun. In fact we kick started our blog on this day just over a year ago, when we explored the elliptical shape of our planet's orbit and how significant this was to our understanding of Earth's climate.

Later in the film Helen explains how the proximity of the Earth to the Sun doesn't guarantee warmth - which brings us to the tilt of the Earth (23.4 degrees) - a theme we explore in further detail in episode three.

Throughout this episode Kate and Helen explore the increase in solar radiation and how land and ocean respond to it.

Kate drives over a frozen lake in Canada with an ice road trucker in one of the coldest places in that region and learns how important this ice formation is to connecting communities.

In this film we also tackle ice ages and how over time, as Earth has repeated it's annual journey, it's climate has changed.

Helen dives under water in Belize to discover how sea levels have risen and fallen over time due to ice age - and explores the three cycles that need to be right in order for another ice age to exist.

What did you think of episode two?

(There are a total of three episodes in this series)

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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 ~ 465'586'300 km: day 181

Noctilucent clouds are a summertime phenomenon which were first observed in around 1885. Noctiluscent [ the name means night shining in Latin ] are high wipsy clouds made of tiny crystals of water ice up to 100 nanometers in diameter. They are the highest clouds in the Earth's atmosphere, occurring in the mesosphere at altitudes of around 76 to 85 kilometers (47 to 53 mi).

Clouds in the Earth's lower atmosphere form in a process called nucleation when water gathers on dust particles, but Noctiluscent clouds also form directly from water vapour as well as forming around on dust particles. It is unclear where the dust or water in the mesosphere comes from but it's thought that the particles may be from dust from micrometeors, although some scientists think dust from volcanoes may also be involved. The source of the water is equally unclear as the mesosphere contains very little moisture - approximately one hundred millionth that of air from the Sahara desert but it's possible that the water comes from lower in the atmosphere or from chemical reactions in the upper atmosphere. This water vapour freezes directly into ice crystal to form the clouds in the thin upper atmosphere when temperatures drop to about -120 °C (-184 °F).

For many years Noctiluscent clouds were a very rare sighting, but over the past 20 years they have become more common. Originally confined to the higher latitudes they are increasingly observed in lower latitudes nearer the equator. So why are they becoming more common and reaching lower latitudes?

Nasa's (Aeronomy of Ice in the Mesosphere) which launched in 2007 was set up to study the Noctilucent clouds and to answer these questions.

Dr James Russell at the University of Hampton explains to me some of the findings of the AIM mission (James Russell III of Hampton University, Hampton, Va. Is AIM's principal investigator):

"Noctilucent clouds are the highest cloud in the Earth's atmosphere forming in the mesosphere at high altitudes (approximately 76 to 85 kilometers, or 47 to 53 miles). It seems odd that they are a summer time phenomenon when they feed off extremely cold temperatures, however as heat warms the air near the ground, the air rises. As it rises, it also expands since atmospheric pressure decreases with height ( temperatures in the mesosphere down past a freezing -210º F (-134 ºC).

We are still unsure exactly why they are increasing in lower latitudes or showing up brighter, they are like a geophysical light bulb, you go from no clouds to full formed clouds in days. This may be due to a sudden change in temperature at the altitude that these clouds are formed. They form in an atmosphere with 100 times lower pressure than at earth surfaces.

During the summer season the temperature stays very low at the poles. For a long time we thought the increase in frequency was a result of temperature decrease but now our research is leaning more to water vapour. Increase in water vapour increases the frequency of clouds. The primary reason for more water vapour at higher altitudes is methane which we are most likely responsible for.

Our research still has far to go however. We have been at solar minimum whilst the AIM mission has been out. Heating is different and dynamics is different so we need to continue our research for a full solar cycle."

The mission has now been extended until 2014 and Dr Russell thinks that the additional research may show a link between frequency in Noctilucent clouds and human activity and that this data may prove helpful to climate scientists investigating climate change.

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