After lava erupts from a volcano and is flowing in a lava channel, sometimes the top of the lava cools down and becomes hard, but there is still hot lava flowing underneath.
This makes a tunnel in the ground, called a lava tube.
![lava tube](https://upload.wikimedia.org/wikipedia/commons/thumb/6/62/Thurston_Lava_Tube%2C_Big_Island.jpg/640px-Thurston_Lava_Tube%2C_Big_Island.jpg)
(from: wikipedia - lava tube)
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