QUESTION:
You were also talking about Neptune having an increase in
atmospheric pressure as you descend into the planet's atmosphere.
Being an avid SCUBA diver I understand that atmospheric pressure
is very low high up in the Earth's atmosphere, and gets higher
as you descend to sea level, then as you plunge underwater the
atmospheric pressure increases dramatically as compared to in
air.  Is this the case with Nepture also?  Since the atmospheric
pressure in Nepture increases so dramatically, does that mean
that the atmosphere is so thick it would be like being underwater
here on earth?

ANSWER from Heidi Hammel on May 30, 1996:
The analogy -- that being deep in Neptune's atmosphere would be
like being deep underwater -- is a very good one (as long as you remember
that Neptune's atmosphere is made of hydrogen, not water).  At Neptune's
cloud tops, the pressure is probably just a little bit less than 1 bar,
which is about the same as the atmospheric pressure here on the Earth's
surface (the very air you are breathing).  By the time you got only about
70 miles below Neptune's cloud tops, the atmospheric pressure would
100 times greater.  Remember, Neptune is a big planet (nearly 31000 miles
in diameter, compared with Earth's 8000), so a depth of 70 miles is only
a tiny fraction of depth of the atmosphere.  The pressure just keeps
going up as you go deeper.  Pressures near the center of Neptune (and
the other giant planet) are measured in megabars - millions of bars!