noun
lava tunnel
A natural tunnel formed by flowing lava; used in geology and volcanology.
ハワイには多くの溶岩トンネルがある。
There are many lava tunnels in Hawaii.
溶岩洞 is a more general term for lava cave, while 溶岩トンネル specifically emphasizes the tunnel-like shape.
Compound of 溶岩 (lava) and トンネル (tunnel), a straightforward loanword hybrid.