The ocean is deep. In fact, most of it is deep. Officially anything deeper than just 200 metres is considered the “deep sea”, but the average depth of the entire ocean is about 3.5km and the deepest point – the Challenger Deep in the Mariana Trench, in the western Pacific – is a little short of 11km down. That means that most of the living space on Earth is in the deep sea.
We scientists like to categorise things and the ocean depths are no exception. Depths from the surface to 0.2km is known as the “littoral zone”, from 0.2km to 3km, the “bathyal zone”, and from 3km to 6km, the “abyssal zone”. Anything deeper than that is the “hadal zone”.
The hadal zone is largely comprised of deep trenches caused by tectonic plate subduction that drive the vast abyssal plains steeply down to depths of 11,000 metres in places. But even here, animals thrive, blissfully unaware of how little attention they receive. Here’s an insight into their incredible world.
1. The kingdom of Hades
The term “hadal” comes from “Hades,” which refers both to the Greek kingdom of the Underworld and the god of the Underworld himself, Hades (brother of Zeus and Poseidon). The term can also mean the “abode of the dead”. In modern times, Hades is seen as evil, but in mythology he was often portrayed as unreasonably “stringent” rather than actively malicious. Interestingly, he strictly prohibited the inhabitants of his dominion to leave, which is a rather apt analogy for hadal fauna, as these species are often confined to trenches and are rarely capable of going elsewhere.