Pressure reduces the melting point of ice
Higher pressure lowers the melting point of ice -- unique among common substances.
Scientific Explanation
For most substances, the melting point rises with pressure: the solid phase is denser than the liquid, so pressure favors the tighter packing and makes melting harder. With water, the situation is exactly reversed. Ice Ih is less dense than liquid water — which is why ice floats. Increased pressure therefore favors the denser liquid phase and lowers the melting point.
Quantitatively, the melting point drops by about 0.0075 degrees Celsius per atmosphere of added pressure. At a pressure of approximately 210 megapascals (about 2,100 atmospheres), the melting point reaches its minimum at roughly minus 22 degrees Celsius. At still higher pressures, other ice phases (ice III, V, and beyond) form, and their phase boundaries have different slopes.
Thermodynamically, this behavior is described by the Clausius-Clapeyron equation: the slope of the melting curve in the phase diagram is proportional to the ratio of entropy change to volume change. Since water decreases in volume upon melting (a negative volume change), the melting curve has a negative slope — unique among common substances.
Everyday Relevance
This phenomenon is often cited as the explanation for ice skating — however, the pressure under a skate blade is far too low to significantly depress the melting point. The actual gliding ability relies on a thin, quasi-liquid layer on the ice surface that exists even without additional pressure.
The pressure-induced melting point depression becomes practically relevant in glaciers: at the base of large ice masses, the enormous weight creates pressures that can actually lower the melting point. The resulting meltwater acts as a lubricant and contributes to glacial flow. This effect also plays a role in regelation — the process by which a wire slowly passes through a block of ice.