Hydrothermal vents are powered by the heat of submarine volcanoes.
Water is drawn down into the rock, boils and then steams out again into the cold ocean.
As these parts of the oceans are lined with many vents, particular molecules can go through the vents many times. This repeated hot and cold cycling of water was the vital procedure. But to preserve the product, we need to transfer it quickly to an environment which is pretty cold. This is what happens at hydrothermal vents. Inside the vent, the water is very hot.