In an ordinary ice cube tray, the water freezes inward from each side.  This results in the centers of the cubes becoming cloudy.  When ice freezes unidirectionally, as in a lake, it remains clear, as impurities are forced downward into the reservoir of liquid water beneath.

I am designing an ice maker that uses this to make clear cubes.  The design consists of a silicone mold that rests in an insulated tub.  The mold is partially open to the air, causing heat to be lost much more rapidly from the top.  The water will then freeze top to bottom.  The tub has open space beneath the mold, which will act as a reservoir like the bottom of a lake.