The main RuneScape Wiki has an article on: Chest key (Digsite)
The Old School RuneScape Wiki also has an article on Chest key (Digsite).

A Chest key is a quest item used in the Digsite quest. It is used to unlock a chest containing Unidentified powder, which can be mixed with ground charcoal to make Nitroglycerin.

When the player uses the key, they will drop and lose it.

Players who wish to obtain multiple keys can do so by continuing to beg the underground Workmen for them. Players do not have to use to the drop trick to obtain more than one key at once.

Players who have completed the quest may obtain this key after the quest by showing the Archaeological expert another Talisman of Zaros. Then using the Permission scroll on a Workman. You will again be granted access to the winches. Players will however, need to attach another rope to the winch before they can go down them again

Community content is available under CC-BY-SA unless otherwise noted.