"Jesus Martinez-Frias, a planetary geologist at the Center for Astrobiology in Madrid, pioneered research into megacryometeors in January 2000 after ice chunks weighing up to 6.6 pounds (3.0 kg) rained on Spain out of cloudless skies for ten days.
The process that creates megacryometeors is not fully understood, mainly in relation with the atmospheric dynamics necessary to produce them. They may have a similar mechanism of formation to that producing hailstones. Scientific studies show that their composition matches normal tropospheric rainwater for the areas in which they fall. In addition, megacryometeors display textural variations of the ice and hydro-chemical and isotopic heterogeneity, which evidence a complex formation process in the atmosphere. It is known that they do not come from airplane toilets because the large chunks of ice that occasionally do fall from airliners are distinctly blue due to the disinfectant used. However, others have speculated that these ice chunks must have fallen from aircraft fuselages after plain water ice accumulating on those aircraft through normal atmospheric conditions has simply broken loose. However, similar events occurred prior to the invention of aircraft...
More than 50 megacryometeors have been recorded since the year 2000. They vary in mass between 0.5 kilograms (1.1 lb) to more than 200 kilograms (440 lb). One in Brazil weighed in at 220 kilograms (490 lb)."