Here's something strange and beautiful from the archives of the Monterey Bay Aquarium Research Institute, whose researchers demonstrated that the peculiar appearance of deep-sea fish Macropinna "barreleye" microstoma is the result of its transparent head. The structure is fragile and invariably destroyed when brought to the surface, which is why we did not know about it until they sent a sub down to check it out.
Video from MBARI's remotely operated vehicles (ROVs) [was used] to study barreleyes in the deep waters just offshore of Central California. At depths of 600 to 800 meters (2,000 to 2,600 feet) below the surface, the ROV cameras typically showed these fish hanging motionless in the water, their eyes glowing a vivid green in the ROV's bright lights. The ROV video also revealed a previously undescribed feature of these fish–its eyes are surrounded by a transparent, fluid-filled shield that covers the top of the fish's head. Most existing descriptions and illustrations of this fish do not show its fluid-filled shield, probably because this fragile structure was destroyed when the fish were brought up from the deep in nets.
Barreleyes is believed to dine on "animals captured in the tentacles of jellies", and you can see more of it at MBARI's gallery and in the video below. The researchers, B. H. Robison and K. R. Reisenbichler, published a paper, Macropinna microstoma and the paradox of its tubular eyes, though only an abstract is available online.