55 million years ago, a giant island burst out of the ocean off the northern coast of what is now Scotland. It existed quite happily for a million years or so, and then disappeared, only to be discovered again, now buried a mile beneath the ocean floor.
A million years sure sounds like a long time, but from the perspective of our 4.5 billion year old planet, it's barely a sneeze. It's definitely not much time for a huge island, practically a continent, to be created and then consumed again.
As you can see in the picture above (created from sound waves bouncing off of different types of rock beneath the ocean floor), the place was covered in mountains, deep valleys, and a complex river system. If it was still around today, we'd probably all be visiting there and oohing and aahing over the magnificent scenery. So what happened?
Scientists think that this mysterious island both rose and fell thanks to what's called a mantle plume. A mantle plume is basically a huge upwelling of magma that pushes up towards the surface with so much pressure that it's capable of raising up the entire ocean floor into a new island, or a whole series of islands. Iceland (well known for having more than its fair share of volcanoes) is on top of a mantle plume. Hawaii is also on top of a mantle plume that's been active periodically, creating a chain of islands as the tectonic plate underneath the pacific ocean slowly moves over the spot.
In the case of the Scottish island, it got pushed up to the surface of the ocean by a giant mantle plume, and when the plume subsided after a million years or so, the island sank back down again. This may also have had the side-effect of releasing huge amounts of methane into the atmosphere, triggering a drastic phase of runaway global warming. So this is all sort of a big deal, and scientists are trying to get it all nailed down so that they can better understand both what causes such events, and how they fit into our planet's geologic and climatological history.