Mesopelagic Zone

The mesopelagic (Greek μέσον, middle) (also known as the middle pelagic or twilight zone) is that part of the pelagic zone that extends from a depth of 200 to 1000 metres (~660 to 3300 feet) below the ocean surface. It lies between the photic epipelagic above and the aphotic bathypelagic below, where there is no light at all.

Physical conditions
Although the temperature varies less at any one height than the epipelagic, the mesopelagic is the location of the thermocline; and in warmer regions of the world, the temperature varies from over 20 °C (68 °F) at the top to around 4 °C (39 °F) at the boundary with the bathyal zone. Water generally moves slowly in the mesopelagic with a residency time of about a century though a variety of animals move vertically through the zone on a daily basis and various debris sink down in relatively short time frames.

The region was initially discovered by American researchers during World War II in 1942 during anti-submarine research with sonar. Sonar at the time could not penetrate below this depth due to the large amount of creatures obstructing sound waves.