lava cave
A natural cave formed by flowing lava, often a lava tube.
There are many lava caves at the foot of Mt. Fuji.
鍾乳洞 is a limestone cave formed by water erosion, while 溶岩洞穴 is a lava cave formed by volcanic activity.
Compound of 溶岩 (lava) and 洞穴 (cave).