Are you sure they were man-made?

There are many lava tunnels on the island. Like Mauna Loa in Hawaii, Teide is a basalt shield volcano.

There is a good example of a lava tunnel visible from Los Gigantes beach.

I know Masca used to be the lookout point for pirates etc years ago, don't know if this helps.

Jackie

Yes, I am sure that they are hand made. Usually nobody paid attention, I guess, since they are a bit aside from the regular path and then they are quite high. But there is something like concrete gutters for water and tunnels which are made inside cliffs for these gutters. Some tunnels are 5-10m long and some are even longer. And the height of the tunnels is about 1.60-1.80m. The tourist went through till the tunnel turned left into the cliff, then he decided he does not want to go further, found himself on the top one of the cliffs and descended by a cliff wall risking his life (he estimated the cliff high as approximately 60m at this point). And I remember Alf was mentionening this tunnels and that there were fatal accidents during their building but at the time of our trip I did not paid much attention...

I know about pirates but I am not sure they had concrete to make such gutters...

It is really interesting to learn who did this gutters/tunnels and what for. If anybody would go in the Masca gorge with touring group or if anybody has a contact to one of the guides, could you ask the guide about these tunnels, please?

They sound a little like the Levadas found on the island of Madeira, which were made for irrigation purposes. These are like small concrete channels running through the mountains and take the water from the wetter, higher ground down to the lower levels to aid cultivation. They often disappear into tunnels cut into the cliffs and mountain-sides. The ones in Madeira have maintenance paths running alongside them and have become popular with walkers because they are level and make walking through the mountains easier.