A few years ago, Planet Oceana was hit by a massive asteroid. This would cause a mass extinction similar to the one 66 million years ago on Earth, But Oceana is a Super-Earth. And she's also a water world. So, that impact was pretty small. Either then, it caused a chain of events. Such as the formation of a ring system, and the complete flooding of the entire planet. This is due to that asteroid being a carbonaceous chondrite, containing HUGE amounts of water in it. No landmasses exist on Oceana anymore save for some volcanic islands that are rare. Enjoy this animation.
