This formed a roof over the river, enclosing it in a lava tunnel or tube.

Some people have suggested that future colonists on Mars could use lava tunnels as shelters.

After an hour of tortoise-gazing in the equatorial sun we head off to the island's lava tunnels.

We continued to burrow through the lava tunnel as before.

Some were still crying but one was shouting that it was okay, there were no more lava tunnels.

These flows created lava tunnels which still exist under the ground surface.

The lava tunnel is at a ten-minute walking distance to the south of Madelena.

And yet here we stand, in a lava tunnel on the planet Mars.

If only they had known about these lava tunnels.

Such lava tunnels are quite common, and they can be most useful.