Given that they spend a lot of time at the lightless bottom of the ocean exploring "black smoker" geothermal vents, this was visually phenomenal. I heard somewhere that they used a new lighting system, and the color at depth was excellent - the teeming life they found down there was astonishing, and the mystery of the "patterns in the sand cores" was also quite interesting - possibly more so because the actual creatures responsible for the patterns had not, by the end of the movie, been located.
As a professional scientist in another field, I found this riveting - and so did my six-year-old. Can't say that about too many nonfiction films out there! One of the best features is that the film does go into detail, but mixes the narrative detail in with the stunning visuals in such a way that while older viewers are listening to the science as well as looking, the kids are looking at all the cool (well, hot, actually) stuff and may also be listening.