But by far the biggest challenge for Smith and his group was plutonium, a metal hitherto available only in microgram amounts, and whose properties were largely unknown. It was initially assumed that plutonium would have properties similar to that of uranium, but this assumption turned out to be invalid. Plutonium proved to be "the most complicated metal known to man". There were found to be six allotropes of plutonium, more than any other metal, and its melting point turned out to be hundred of degrees lower than uranium. The metallurgists found that at around 125 °C, plutonium expanded in volume by 20 percent, which is unusual.