From there it we were able to hike 0.8km completely underground. The terrain was very rocky - some of the people on our tour were expecting flat ground with rocks on the sides... that wasn't the case at all. It was much more like the caves you would find in the Rocky Mountains, except of course for that you are in a tube made entirely of hardened lava.
In the lower reaches of the cave the ceiling is made of silver - the lava melted silver in the ground and as it was less dense than the lava itself, it rose to the top. However the silver did not need to be as hot as the lava to flow (about 1800 degrees as opposed to 2000 degrees C), so it actually helped the lava continue to flow as it cooled.
The furthest reach we got to was where the lava dived deep into the ground. The flash on my camera didn't reach the bottom, but it was about a 30-40ft drop, and then the tunnel changes direction and goes underground for another 400m (or so we were told).
The guide explained how the lava acts like water when it is flower, taking the easiest route. The lines on the wall show the different flows of lava as the pressure built and receded.
At one point during the tour the guide got us to all turn off our flashlights. It was complete dark - you couldn't see a hand right in front of your face - couldn't tell the difference between having your eyes open or closed. Then he got us to make as little noise as possible and just listen. All that could be heard was the water dripping in the cave/lava tube. It was surreal.
When we got back to surface it was a good thing and a bad thing - on one hand it was nice to be back in the daylight, but on the other hand it felt like the whole thing was over far too fast.
Tomorrow we will be going on a Super Jeep tour of the Golden Circle, and snowmobiling on a glacier.