Check out these Undersea Cable Maps for Africa by Steve Song of the Shuttleworth Foundation. The latest one is an outlook of what cables could provide Africa with data by 2013, indicating the ones already active:
Sub-saharan Undersea Cables in 2013 – maybe (version 29) by Steve Song on Flickr, CC-by
The English Wikipedia has an interesting article on submarine communications cables and their history. On the question of the environmental impact of undersea cables, it cites this report (page 29 forward).
The Guardian published a World Map of Undersea Cables in 2008 including context info. Greg’s Cable Map mashes information from Wikipedia and the Web and is regularly updated. It’s a pretty cool and detailed map, giving you the names of cables, landings, and more info on each one of them.