What causes ice to be slippery underfoot when frozen?

The slipperiness of ice is primarily caused by a thin liquid-like layer that exists on its surface, even at temperatures well below freezing. This quasi-liquid layer forms because water molecules on the surface have fewer neighboring molecules to bond with, making them more loosely held and mobile than those deeper in the ice structure. Pressure and frictional heating from walking play smaller roles than once thought. Early theories suggested that pressure from a skate blade or shoe lowered ice's melting point, creating a lubricating film, but calculations show this effect is too small to fully explain the phenomenon. The pre-existing surface layer is now considered the dominant factor, reducing friction and allowing surfaces to slide across ice with little resistance.