Surfing a few espresso and "moka" explanations, it does sound like
espresso machines operate at below-boiling-point temperatures --
around 90C is claimed to be optimal, _well_ below boiling at the very
high pressures involved. Most web sites get it right, that _water_ is
being forced through the grounds, but some mistakenly claim it is
steam.

Plenty of sites describe the "moka" contraptions as operating at
higher temps, and given that the water is reaching its boiling point
at slightly higher than atmospheric pressure, they must be. So the
wikipedia explanation for moka machines must be correct.

The claims of espresso sites that temps much higher than 90C ruin the
brewing process seem to contradict the moka brewing process -- the
only explanation I see is that the chemistry changes quite a bit at
the espresso pressures. Moka pressures are only slighty above
atmospheric.

Jon.
Not replacing my Mr. Coffee anytime soon...