Source: http://en.wikipedia.org/wiki/Steelpan.htm
Steelpans derived from the playing of frying pans, dustbin lids and oil drums in Trinidad in 1937 as an alternative method for social interaction after other events, such as stick fighting were banned. In order to have more control over pitches, contemporary steelpans are manufactured as such, and no longer use modified oil drums.
A circular steel plate (a solid membrane), which is already tensed, is bulged out into a bowl shape - by hammering during the manufacturing process. Certain areas is tensed even more, by bulging them out even more. The different dimensions of these smaller bulges create different pitches. As other bulges (of different pitches) may also resonate when a particular one is struck, the harmonics sound somewhat dissonant. A pipe-like frame is added that serves as soundbox. As it is circular, the acoustic properties would be similar to those of pipe instruments - except that the air column is not moved with the same force.
Source: http://en.wikipedia.org/wiki/Steelpan.htm
http://www.youtube.com/watch?v=OnGPRQdEPWw