Having had another nice breakfast of rolls and apple juice, I was picked up at dawn (that is, 10.30 in the morning) and taken to the Blue Lagoon, the big geothermal spa and ultimate Iceland must-do.

My favourite thing about the Blue Lagoon is that it's waste water from the power station next door. Svartsengi is a geothermal power station. The sea water comes up from below the ground at 200-300 degrees under high pressure, is used by the power station to generate electricity, then flows over to the Lagoon, which was dug out of the solid black lava, once it's cooled enough to not take our skin off. This place is not natural.

It is paradise. I had done a lot of reading before I went there. I knew about the naked showers before you're allowed in, I knew about the drying effects of the water on hair and to put conditioner on it before going in, I knew about the electronic bracelets to open the lockers and pay for snacks (an ingenious system - I love it), I knew about the uneven lagoon bottom, rocky in places, silty in others. The one major thing no one had mentioned is how incredibly salty the water is. You don't have to get it anywhere near your mouth to be able to taste it.

I had locker 55 - 56 would have been easier to remember but it was a bottom locker - and I managed to figure out the bracelet lock - the locks are not on the individual lockers, they're for a block of about four. I also manage to knock myself on the head with the corner of the locker door which didn't bother me too much until I discovered two hours later I'd created a huge lump on the side of my head by doing so.

Next was the infamous naked shower ordeal - not actually so bad because a lot of the showers have doors. They don't lock (or even close all the way) and they're opaque glass but they are doors.

Now I can go into the water. I had read about this - you can either go through the door and brave an Iceland December before scrambling into the water or you can go into the little pool to the left from where there is a door to the outside, meaning you don't have to leave the water. I chose this option every single time. It was a hideous day. The car park is a little distance away, down a lava-sided path. I hadn't bothered with my three hundred layers today - I came out in t-shirt, fleece and coat and I hadn't done up the fleece or coat. Getting from the bus to the door was horrible - howling wind and heavy gale, trying to hold my hood up with one hand and hold my coat closed and hold onto mittens and ticket with the other while keeping my head down so I was mostly blind and rapidly losing feeling in my fingers. So it wasn't a nice day but better to have a rain/snow storm while I was drifting in warm water than while I was doing something else.

It's very nice to drift in warm water while Iceland throws a storm over your head. You do have to keep your shoulders below the water and dip your ears in every now and then. And because of the wind you had to swim around facing the building because if you turned round, you got your face scoured off by high-speed sharp rain.

I investigated the lagoon, found the hottest spots - some painfully hot. Found the white silica mud and smeared that on, then took shelter inside to investigate things like the cafe where I also investigated the space-age coffee machine and the bracelet paying method by having a cup of hot chocolate and then the relaxing room, where I relaxed in a nice chair by drawing the view although as it disappeared inside low-hanging clouds while I was drawing I had to make it up a bit. The sauna and steam room turned out to be accessible via the lagoon - I'd have to go outside for them. I also investigated the towel-robe-locker conundrum. The changing area is for changing. You have to get dry in the shower area before going to the changing area. I had left towel and robe in my locker so I dripped a bit, then sneaked back to my locker. The best way, I concluded, was to leave the robe in my locker where no one else could walk off with it and leave my towel in the racks in the shower area. It's fun to walk around in a white robe. Very luxurious.

I ventured outside again and found the sauna and steam rooms. My ability to stay in them has improved hugely and they're particularly nice if you've had to climb out of the warm water and scurry across decking to them during an Icelandic December rainstorm. Then you don't need the cold sprinkle afterwards - just getting back to the warm water gets you frozen.

I had an unusual lunch of too-sweet apple juice, a mini Babybel and half a packet of salt & pepper crisps which nearly burnt a hole in my tongue - I had to sneak the rest back to my locker using the big sleeves of my luxurious robe.

In the afternoon I drifted from one hotspot to another, used the silica another couple of times, had another cup of hot chocolate and liberated four little pebbles - I'd noticed the bottom was a bit pebbly in places and when I scooped some up with my feet to look at them, they turned out to be black and white speckled lava pebbles so they also got sneaked back to my locker.

At four o'clock, a lot of people left, presumably a tour group who took up all the showers for quite a while and then made the place much quieter. I went back outside. It was getting dark by now and although the novelty of being in an outside pool in such weather hadn't actually worn off, the novelty of being in an outside pool in such weather in the dark had come along too. It could have done with being better weather - there were quite big waves in the pool, you had to swim backwards a lot of the time and I was going from hotspot to hotspot and by then, I knew where every single one was.

Then at five on the dot, the wind dropped and the rain stopped and suddenly it was a different world. We could swim in whatever direction we wanted without losing any eyes, the surface was smooth, steam rose off the water and hotspots appeared all over the place - like right in the centre which had previously been a particularly cold spot. And by "particularly cold", this is by Blue Lagoon standards. Still easily warm enough considering it's Iceland, December and raining. Now it was really really fun, much more peaceful, much warmer and even more amazing. I swam up to the far end, where I hadn't been before and watched some strange red glowing lights in the sky. Probably not the Northern Lights - it was too early in the evening and anyway, they're usually green. But they streaked the sky for a few minutes before disappearing which is why I'm disinclined to think it was the power station or the airport glowing.

I could have stayed there forever. I could have spent the last three days there instead of seeing the country, especially once the weather cleared up. But my bus went back to Reykjavik at 7pm so I had to drag myself out. In order to maximise my time in the lagoon, I didn't bother with a hair-cleaning shower - I planned to do that when I got back. I did dry my hair though, or I attempted to. You couldn't tell the difference. But while I was doing that I realised my ears were full of silica.

After I'd got back to the hotel and phoned home only to discover afterwards, judging by the state of my phone, either my ears were still full of silica or my hair was. I looked in the mirror and discovered the stuff had dried nicely all around my ears and made a pure white crust. So that was nice. Still, my ears are nice and deep-cleaned now. And my hair is conditioned and soft despite the lagoon's best attempts to turn it into straw.

I am coming back here. I have fallen in love with Iceland in a way I never have with any other country. I am coming back.