It is old. I don't thnik it aged very well, as a matter of fact I'm sure it aged way less good than its sequels. It isn't very well written when it comes to characters or dialogs.
Yet, it has brought so much. It is still fully playable nowaday du to the scratch, its simplicity in level and gun design. Some ideas are still great : vehicles, vast levels, modern health bar, certain guns like the needler or plasma grenades... Halo has brought the scratch technology, which every shooter use since. It is still a lesson in enemy design and world design nowadays.
This cave isn't a natural formation.