Comment by robocat

Are you guessing based on installations in other countries, or do you know that?

I would presume that volcanic rock is difficult to put a trench down through. But I am interested to know.

Edit: I found a good article with pictures of the equipment used about water jets to trench in soft seabeds: https://www.mdpi.com/2077-1312/8/6/460/htm It mentions cable ploughs and mechanical trenching machines. I would presume trenching in rock is sometimes required near-shore.