From the Proteus press kit:
Proteus is a game about exploration and immersion in a dream-like island world where the soundtrack to your play is created by your surroundings. Presented and controlled like a classic first-person shooter, the primary means of interaction is simply your presence in the world. The procedurally generated islands are home to creatures natural and imagined, tranquil valleys and ruins with magical properties.
Just finished playing it and I'm not quite sure what to say, other than "it's really good". It's a sublime experience (I'd hesitate to call it a "video game"). I took a few screenshots.
If nothing else, I recommend it.