There will be a minor spoiler in this guide relating to an event you go through near the start of your journey and arrive at tyr's temple. It's minor, but will be tagged none the less.
The lake of nine is located in midgard, tyr's temple is in the centre.
You can earn this trophy and explore each point by Docking at the islands around the edge of the lake. You can do this after
Working clockwise from the northern most point, here are all the locations you need to visit:
You can visit these locations in any order, just make sure you discover them all.
- Light Elf Outpost
- The Mason’s Channel
- Council of the Valkyries
- Stone Falls
- Buri’s Storeroom
- Cliffs of the Raven
- Lookout Tower
- Isle of Death
- Iron Cove
- Forgotten Caverns
- Ruins of the Ancient
Once you’ve visited all of these locations at least once, the ‘Beneath the Surface’ Trophy will unlock and you’ll have opened up the entirety of the map.
I've been notified that returning to lookout tower will unlock the achievement in case that it doesn't pop up.