Here is a good article on the causes of the great depression, and basically it was so great because every country in the world was affected. Once the U.S. economy fell the European economies which where dependent on U.S. investors fell. Then all of the countries that were dependent on trading with the U.S. and Europe collapsed.
Unemployment rose to 24.9% (nearly 3 times as that of 2008-2009) and in some parts of the country it rose to 80%.
The world economies where down so low that it took a world war to bring them out of it.
