The World Ocean's midwaters contain the vast majority of Earth's vertebrates in the form of mesoand bathypelagic ('deep-pelagic,' in the combined sense) fishes. Understanding the ecology and variability of deep-pelagic ecosystems has increased substantially in the past few decades due to advances in sampling/observation technology. Researchers have discovered that the deep sea hosts a complex assemblage of organisms adapted to a “harsh” environment by terrestrial standards (i.e., dark, cold, high pressure). We have learned that despite the lack of physical barriers, the deep-sea realm is not a homogeneous ecosystem, but is spatially and temporally variable on multiple scales. While there is a well-documented reduction of biomass as a function of depth (and thus distance from the sun, ergo primary production) in the open ocean, recent surveys have shown that pelagic fish abundance and biomass can 'peak' deep in the water column in association with abrupt topographic features such as seamounts and mid-ocean ridges. We review the current knowledge on deep-pelagic fish interactions with these features, as well as effects of these interactions on ecosystem functioning. We highlight the recent discoveries from the Mid-Atlantic Ridge (via the Census of Marine Life field project MAR-ECO) that were presented at the international symposium “Into the Unknown, Researching Mysterious Deep-Sea Animals,” hosted by the Okinawa Churaumi Aquarium, Okinawa, Japan, Feb 2007.