Last weekend I was on an island in the Howe Sound, and discovered this massive stump and root system washed up on a rocky beach. In all cases, click the photos to see a larger version.
I figure the thing was about forty feet across, and the tree itself might have been three or four feet across at its above ground base. I’ve seen bigger ones on fallen trees in the forest, but you rarely see the whole base of the tree exposed.
It’s a little hard to see, thanks to my lousy iPhone photos, but the stump carved a path through the rocks on the beach when it came aground.
The underside was covered in thousands of mussel shells. They rattled like wind charms in the breeze.
There’s actually a fair bit of plant life on the tree’s underside.
I was intrigued to discover a rusty hook in one of the roots. Was this installed to enable someone to tow the huge chunk of wood somewhere? Because it was upside down, it was difficult to tell whether the tree had been cut down–the likelier option–or had fallen over of some more natural cause.
DarrenBarefoot.com 
Definitely shows how land organisms can “raft” from one place to another, spreading populations to isolated islands and so on.
If it was cut, it was probably cut AFTER it had toppled. Otherwise, the roots would probably still be firmly in place in the ground.