Stewart,

Apparently this urban legend could be true.  Folks in the know say it is
possible (but unlikely) and that you shouldn't boil water in the microwave
without a wooden spoon, or something in the water.  I thought of this the
other day as I was boiling a bowl of water with shells in it trying to get
the opercs lossened up.  There was also a couple of rocks in the dish and
one of them must have had water in it as it blew up.  Bit of a mess but no
shell breakage.  Anyway, if you want the total answer about the microwave
check out:  http://www.snopes.com/spoons/faxlore/microwav.htm

Tom Eichhorst in New Mexico, USA