There are several lava caves in Iceland. Raufarhólshellir is one the longest and the best-known ones. By visiting this cave you can get a feeling of inner workings of a volcanic eruption that occured about 5200 years ago.
The cave is about 1350 meters long and not easy to to explore if you do not have a powerful light source. You also need good shoes, a hat or a helmet and gloves.
Raufarhólshellir is located just 30 minutes from Reykjavík and it can be accessed in every season.