Answer:
It looks like children working in a coal mine so i guess Gases, Mines could collapse, and possibly get sick from fumes or something.
Copper sheathing<span> is a practice of protecting a under-water hull of a ship or boat from the corrosive effects of salt water and bio fouling through the use of </span>copper<span> plates affixed to the outside of the hull.
It was pioneered and developed by the Royal Navy during the 18th century.
Hope this helps.</span>