Temperature
A photo of an iceberg floating in the cold seas of the southern ocean, taken from HMS Challenger
How cold are the depths of the oceans where sunlight never reaches?
The surface temperatures of the oceans range from around 40°C in shallow tropical lagoons to -1.9°C, the freezing point for sea water, in polar regions.
Most of the water in the deeper layers is very cold. Even beneath the hottest tropical regions the water at a depth of 2 to 3km rarely rises above 4°C. Deep sea creatures have had to adapt to far lower temperatures than creatures living on land.