Today I actually had a lovely relaxing day. Pickup wasn't until 9.30 which gave me hours to get up and get packed and get ready to go, since I woke up at around 6.30.

What I actually did was follow in the footsteps of Jules Verne or various Bond villains and I descended into the magma chamber of a volcano called Þríhnjúkagígur (Three Peaks Crater). This is the only volcano known in the world to have an empty magma chamber - it seems that after its last eruption, the magma just drained away, leaving a bit funnel-shaped hollow inside the volcano. Although apparently not all the magma drained away - what was left solidified eventually and formed the floor we were going to walk on. They think the magma chamber actually goes much deeper, it's just full of magma.

Pickup, when it happened, was by Iceland Excursions, which I'd sort of suspected it would, although I'd booked directly with Inside the Volcano. So it was the usual ritual of being taken to the ticket office to swap my paper for a real ticket and then find the correct.

I'd been expecting us to head out towards Hafnarfjorður so it was a bit of a surprise to find us going on the Hveragerði road instead. Our guide, Kristian, pointed out our volcano on the horizon and then pointed out Hengill, the one with the power plant. The usual - this is a geothermal power plant, they dig boreholes and use the steam from them to turn turbines to generate a lot of electricity but he added a bit of extra. When the steam's done its job, they pump the water back into the volcano and this apparently has the side-effect of causing small earthquakes. They don't mind that too much on the west side of the volcano because no one lives there but there's a small village on the east side and they "are not so happy" to have regular earthquakes.

We turned off the road before Hengill and went up into the Blue Mountains, up a very nice smooth bendy road. I couldn't make sense of this - they didn't build the road for the volcano, they couldn't and if they had, surely it would reach a bit closer. But why was there such a good road up here? And it's a very good road - better than the bits of the Ring Road I've been on and better than the highway to the airport and it didn't make sense up in these mountains.

The mystery was solved when we reached our destination. The road goes up to what passes for a ski resort in Iceland and we parked in their car park. It is of course empty this time of year - at the moment, the ski slopes are green-streaked lava heaps.

We left the minibus and started our two mile hike across the lava fields. I'd thought I'd be the one struggling along at the back but I'd looked at people's feet as they boarded the minibus and there were a lot of trainers and even a pair of pretty little black leather heeled boots of the kind you wear for a shopping trip. It wasn't a hugely difficult walk but it had been spitting a bit when I left so I was wearing my big purple raincoat and very quickly that got too hot. At our first stop, that came off. Our next stop was at a crack in the ground - a gap maybe three or four feet wide where the North American and European plates are moving apart. This happens all the way across Iceland, one notable location being where it's around five miles apart at Þingvellir, which is so far that you can't really see what's going on. Here you could either walk across where muddy boulders filled it or leap across. Everyone took the easy step and although it was just a gap in the rocks, that was my first wow moment of the day. I have stepped from the edge of one tectonic plate to the edge of another. I've driven across from one side to the other many times, since Reykjavík is on the North American and everything east of about Hveragerði is on the European plate and I've flown over it but I've never crossed continents like that.

Kristian talked about the big plan for Þríhnjúkagígur - to dig a tunnel under the lava, drill into the side of the magma chamber and create a viewing platform there. The main concerns with this project are environmental - which led him onto the subject of renewable energy. They make a lot of use of geothermal and hydro-electric power here but haven't had much luck with wind turbines. Apparently they just blow away. There was a farmer who was very pleased with his shiny new extra-tough extra-secure turbine, who generated a lot of electricity with it and who woke up one day to find it gone because the winds are too strong.

We kept on walking and after under an hour, we were at the containers that serve as Inside the Volcano's base. Five of our ten-man group was immediately got into climbing harnesses (apparently beyond certain members of the group) and then helmets with lights and within five minutes of arrival, we were standing at the top of the crater. We stopped for a couple of quick photos and then we had to "walk the plank", clipped onto a wire safely and climb down into the basket, or window cleaner's lift, also clipped in. We weren't allowed to take photos here - all cameras had to be safely around necks or wrists so they wouldn't get dropped down the abyss. I'd had the foresight to bring the carabiner off my keys and the neck string from my waterproof case and with my camera safely tethered to my harness I was able to take as many photos as I wanted.

Standing over the hole was terrifying. I'd been Professor Lidenbrock all the way, now I suddenly became Axel. It was particularly terrifying when we unmoored and began our wobbly descent. At first the rocks were mossy and reddish and we went through a slot so narrow that we had to stop the basket and creep down, with the little rubber wheels on the side of the basket keeping us away from actually hitting the sides. Once we were through there, we could see the bottom below us, or at least, we could see the big lights.

It opened out and the colours began to appear - patches and streaks of red and yellow and green and grey and orange, We reached the bottom, disembarked and were taken beyond the rockfall danger zone so Kristian could explain what was going to happen next. We were allowed to wander freely wherever we wanted, basically. At each side, the chamber dropped down and if we wanted to scramble down the rocks, we could go down to where it narrowed so much you had to crawl, and you could get good photos from down there.

Our group went as one down one side, where we managed to get caught behind the girl who'd come in a fur coat, skinny jeans and trainers and was struggling with the loose slope. Kristian hopped down with us to show us the way until he heard the lift coming and we were left unsupervised. What was I supposed to do? Of course I was going to scramble down the heap of boulders to the bottom. Give or take the fact that some of the boulders were a bit crumbly because they're made of lava, and that some still move, it was a pretty easy scramble. I went all the way to the bottom because you can't turn back halfway, not in a volcano and sat on a boulder taking photos of the yellow and orange dome above me. Then I scrambled up, over the mound in the middle and down the other side. The Australians were there and as I hopped, skipped and jumped down, they said I looked like I belonged here. The bottom here wasn't actually quite the bottom. I scrambled over another boulder and dropped down just a little further and then I could see a small narrow crawl-sized hole below me which I was half-tempted by but there didn't seem to be any way to climb down and dropping in didn't seem like such a sensible idea.

By the time I was climbing back up, it was already time to depart. Far too soon. That's my one criticism of the trip - I could have done with a bit more time inside the magma chamber. But as we went up, I had a better appreciation of what I was seeing. There were people in the chamber as we ascended so I took photos of them getting smaller and smaller below us and photos of the tiny circle of light in the ceiling 120m above us which was the only way in and out.

Oddly enough, once we were back at the top with daylight on us, being in that basket was scary again and "parking" it at the top was a delicate little manoeuvre to get us docked properly with the plank.

Back at the cabins, we had our harnesses and helmets removed and the other four went to eat Icelandic meat soup. I hovered in the doorway until the nice lady ladling out soup suggested I go for a nice walk while I waited, under the lava bridge ( a collapsed lava tube) or up to the broken crater. Off I went, across lava and moss and scrambled down into the little canyon. It was absolutely silent and peaceful and even though there were a handful of people only two minutes away in the cabin and more up on top of the volcano, it was like I was the only person for miles around. I played with the timer on my camera and took a few photos of myself in the lava field, often only getting my legs or myself as a dot in the distance and then climbed out of the canyon to visit the broken crater.

When I wandered back, the others had finished their soup and were starting to venture out onto the lava field too, so I had a drink and wandered off again. We wandered happily for quite a while, waiting for the other half of our group to emerge from the mountain and eat their soup. I went back into the canyon and then went and sat on a slab of lava to stare at the view.

The walk back was easier. We stopped again at the crack in the Earth and I stood on the edge of the plate to have a photo taken as I stepped across. Only that's quite a big step and it's over a gap in the crust of my planet and it was too far and I chickened out and did the easy crossing over the boulders instead.

I'd been chatting with the other English people on the trip and one of them mentioned that Iceland must be brilliant for caving. I was under the impression that it wasn't because although there are plenty of lava tubes, they haven't found entrances to most of them. I don't know if Kristian heard the conversation or if it was just a coincidence but at that moment, he stopped at one of the lava tubes so those of us who were "mountain goats" could hop down into the caves and go through and emerge from a different hole. I happened to have brought my caving light with me so I could have a look at the rock and into the darkness properly. We were making good time so we had plenty of time to play in those tubes. Mostly they were just bridges under the lava, going from one open pit to the next, because they've collapsed in so many places.

The first lava cave was "Long Hole" and the second "Deep Hole" - Icelanders are apparently as imaginative with naming their caves as with naming their volcanoes (Three Peaks Crater, Snow Mountain etc). Long Hole made its way through at least four open pits and was good fun to scramble through. Some of the bridges were long enough to be able to see where the sides were proper lava tubes but mostly they were full of the rubble that had fallen in. Deep Hole wasn't actually much deeper than Long Hole but it needed a bit more of a scramble to get down. And it had a window, another entrance up on the ceiling with a grate over it to stop hikers falling in. It seemed to be growing a layer of bubbles on top of the lava, like you see in limestone caves, because of water action.

We were back at the car park just as the bus arrived. We left and the next group arrived and Kristian headed back across the lava field with them (he does the tour twice a day, as goes another guide) and bearing a birthday cake for Einar, the lift operator, which had been delivered by the driver. Half an hour later were back in Reykjavík. I got dropped off in the middle of town and walked back without getting lost at all, for the first time since I've been here. I was back by quarter to four and stayed in. Tomorrow I have a very very long day so I've enjoyed having an afternoon in.