The thorny tinselfish. This image comes from a USGS research paper (Sulak et al., 2006); I hope they don't mind that I cropped out the title of this picture - it was just "Thorny Tinselfish". They used submarine robots to survey the ecology of a 'twilight-zone' coral reef. The oceans are generally under-studied, in my opinion, but the deeper one goes, the less is known. Almost nothing is known about what's going on deeper than normal SCUBA depths (i.e. around 30m down). The region deeper than about 100m but shallower than about 500m is sometimes refered to as the twilight zone, because of the low-but-not-zero levels of light during the day.Photo credit and the paper here.
Sulak K, Luke K, Norem A, Randall M. 2006. Sequential Video Frame Analysis of Fish Community Structure on Lophelia Reefs - Northern GOM Continental Slope. Presented at FISC strategic review May 9-12 2006, published on-line at http://cars.er.usgs.gov/posters/Coastal_Ecology/Video_Frame_Analysis/video_frame_analysis.html