Balloon attacks?

I'm reading Into Thin Air.  The author keeps talking about the Jet stream.  I realized I didn't know what it is, so I looked it up on Wikipedia.  Basically it's just fast air currents at a certain position in the atmosphere, at the coldest point.  The wind is so fast, jets that fly in the stream can shave 1/3 off their time flying between Tokyo and LA.  The weirdest thing though, is that the Japanese used balloons filled with explosives, positioned in the jet stream, to bomb America during WWII.  Weird...  The article claims there may be some unexploded balloons in the forests of the West coast somewhere.