What are these river/canyon-like carvings in the ocean? Browsing around Google Maps, I came across this off the south-west coast of Ireland



It looks very consistent with the shape rivers and their tributaries might make, but it has me puzzled since it's all underwater!

The main, most "river-like" one is in the middle-left of the picture but there are more (which look more like fjords) at the bottom.

What caused this feature?
The carvings are submarine canyons, a part of the continental slope leading from the continental shelf to the continental rise and ultimately the Abyssal plains. They are a product of :


erosion through currents and 
slumping of the continental shelf


Like other erosive or slumping effects, they can be self-reinforcing, leading to canyon-like structures. The highlighted canyon here is the Gollum Channel system, seen here.