Urban legend - even in an airtight container, without crazy amounts of embalming there would be bacterial decomposition from the inside out (the bacteria that live in your digestive tract). Supposedly there have been attempts to create airtight caskets to prevent the entry of worms and other exterior scavenging creatures, but the bodies eventually are destroyed by the bacteria anyway (and supposedly it's much more gruesome looking of a process).
I remember reading somewhere that the gasses emitted by these bacteria can cause the body to expand to almost 3 times its original size during decompsition - that's pretty gross!
Anyway, here's the debunking link:
http://www.snopes.com/horrors/gruesome/lucy.htm