Sunken Chests is a type of Treasure Chest. It can be merged into Noble Chest. Tapping will create up to 5 random objects from the following contained in Loot13 Sep 2018
17 Mar 2017
9 Nov 2018 I’m not sure if this is a bug or if I’ve missed something, but I just attempted opening some of the chests for the Sunken Treasure achievement,
8 Sep 2018 I would guess that opening each chest is time gated to once per week. got my key, and ten days later i’m going to my sunken chests run.