Lava caves, simply put, are any caves that are comprised of volcanic rock. These cavities are usually formed during the volcanic process be that due to volcanoes emptying their magma reserves or due to the varying cooling rates of the magma creating these caves. Since Iceland is a highly volcanic country there are many lava caves and tunnels to be found and each one offers a slightly different kind of experience in their exploration.
Many of the caves that you can visit in Iceland were formed thousands of years ago during major volcanic erruptions, their interiors contain unique mineral compositions as well as, in some cases, obsidian which can create some dazzling colors. Journeying into these caves requires having some safety equipment with you as well as the acompanyment of a guide, in Iceland there are miles of these tubes and tunnels many of which are yet to be fully explored so it is best to have someone with you who knows their way around.
Typical equipment you will have with you when journeying into the cave is a helmet and a torch. It is also reccomended that you wear warm clothing as the interior of these caves can get quite cold.