How can I get Hidden Keys?
You can earn Hidden Keys by completing orders from the Merchant Ship. 

What can I use Hidden Keys for?
Hidden Keys can be used to open chests in the water at your Home Station, which give valuable rewards. The keys can also be used to unlock Hidden Gates with energy boxes in event locations.

Where can I check how many Hidden Keys I have?
You can see your current number of Hidden Keys in your Storage or by tapping on any Hidden Chest in the water.

How often do chests appear?
You can have up to two chests at a time. When you open one, a new chest will appear after some time. Chests refresh several times a day, so check back periodically to find new ones.