Fontaine 4.6 Exploration (Initium Iani + Precious Chest)

Location is shown in the video. To unlock this era, you have to do the world quest that starts in Petrichor and follow the storyline until you reach this area.
To unlock the chest, just find all the octopus and wait for them to finish their music :) use the echoing conch to make it easier to spot them.
Don't miss the nearby hydroculus too