The Sky Garden is accessible through the Wilderness after having solved the slate puzzle.
The Umbrella effect can be obtained here.
The Organoid stands in the background.
On the rightmost side are the three Maussan Bros watching the nightsky. If you join them, you'll get rewarded with the "Correlations" achievement.
Next to them is a tunnel with a patch of white lilies at the end of it. Interacting with them will make the screen turn white and wake you up. You're rewarded with the "Primal" achievement.