In article .com,
wrote:

I am not a pilot, and as much as my email address sounds like I am a
doctor, I'm not...

That said, I thought someone here might be able to help me out - they
news reports talk about passing out within seconds at high altitudes.
I understand the air is thinner / less O2.

But if I go underwater, there's certainly no air there. And I can stay
down for a while, holding my breath. How is high altitudes different?

It depends on how you get there :-)

If the decompression is sudden then you can't "hold your last breath."
If you try then the air pressure in your lungs will cause them to
rupture. It's called an "air embolism". Scuba divers can get them too
if they surface while holding their breath.

If the decompression is slow then you can keep breathing but your blood
oxygen level will gradually go down and you will gradually lose mental
function. The effect can be subtle enough that you don't realize that
it's happening even as you drift into unconsciousness. Pilots sometimes
get special training to help them recognize and deal with the symptoms
of decreased blood oxygen at high altitude.

In a sudden decompression it is of course obvious that something has
happened, but your "time of useful consciousness" as it's called can be
very short (seconds). If you don't manage to put on an oxygen mask in
that time, or if the oxygen system isn't working, you drift off to sleep.

There is one case of a pilot who actually managed to survive such an
occurrence. The plane was on autopilot when the (human) pilot passed
out and when it ran out of gas the autopilot kept the wings level and
the plane did a gear-up landing in a field.

rg