It is an active volcano with an enormous and very deep crater that occasionally vents out smoke and steam. (dive-the-world.com)
With depths of up to 500m, it is said to be among the world's deepest crater lakes. (florestourism.com)
Scientists now believe that ice collects on the floors of deep craters at Mercury's poles, where it can remain permanently shaded from the Sun and reach temperatures as low as -235 degrees Fahrenheit (125 degrees Kelvin). (public.nrao.edu)