At about what depth does the hadal zone begin?

The hadal zone begins at about 6 kilometres below sea level.

The conventional boundary is 6,000 metres, separating the hadal zone from the abyssal zone above it. This is a useful classification rather than a sharp physical wall: temperature, pressure, geology, and biological communities change gradually with depth.

The term hadal was proposed in the 1950s after expeditions found distinctive changes in deep-ocean life around 6,000–7,000 metres. Some scientists have suggested placing the transition closer to 6,500 metres, but 6 kilometres remains the standard figure used in general oceanographic classifications.

Below this boundary, the seafloor is usually inside an oceanic trench. Light is completely absent, food is limited, and pressure rises to more than 1,000 atmospheres near the deepest sites. Challenger Deep, at roughly 11 kilometres, lies well within the hadal range.