Actually, the waste water mentioned by zharniwhoop (where do these names come from?) is held in tanks in the aircraft. However, the tanks are vented to allow for pressurization changes. The vents are designed to vent only air... but, because they can malfunction, sometimes the waste water mixes with the air venting overboard and begins to freeze. This can become quite a large lump measuring, in some instances, as much as two or three feet in diameter and weighing as much as 50 pounds or more. Usually, this simply melts as the aircraft descends into warmer air but they have been known to break off. Just a few days ago, here in the U.S., one fell off and came crashing through the roof of a house. Fortunately, no one was in the bedroom into which it fell, but the house suffered severe damage. Point is, some large part of this "blue water" is made up of urine, obviously...