Alright so I found the statue room. I guess I should have gone there earlier but whatever. It's about sharing memories or something? But it helps explain what I saw in the mine - masks. Hammock dude is also in the loop and taught me to meditate until the next loop. Handy.
Also found an island on giant's deep with a red creature encased. I couldn't get inside it or interact with it, felt like a waste of time. There was ghost matter there though so I thought it might be something.
Two avenues to explore next time - the path to the observatory, so hopefully I can build something that goes beneath the surface of giant's deep, and also the Moon which I didn't visit yet.
Also there's some ghost matter on hammock dude's island - I was in the corridor containing it when a cyclone hit and I got thrown THROUGH it, barely survived, so thought I could explore what I'm missing there - but then I died upon landing. Half-tempted to try that again