Centuries of eruptions on the Big Island have created hidden networks of passages beneath volcanoes. In December 2016 a large lava delta formed by Kilauea volcano sank into the sea, exposing the mouth ...

Sometimes, a hole in the ground is just a hole. Other times, it’s a portal to an underground cave spanning miles in width and length. You don’t really know what’s inside unless you dive into the void.

Another world greets you just a short 30-minute drive from the Icelandic capital Reykjavik. Raufarhólshellir Lava Tunnel is a terrific example of a lava cave created following a volcanic eruption ...