I've heard it called the "Lizard Brain" -- the primitive part of the brain that tells us "don't do that - you will die!"

There are two specific times that I can recall having to force those fears down:

One was sitting on the platform of the zip line at the scout camp COPE course. 41 feet off the ground, about to trust my life (or at least my non-broken bones) to an awfully thin looking steel cable. Hooching my butt off that platform after looking down at the tiny ant-like people on the ground was very very difficult. <img src="/images/graemlins/wink.gif" alt="" /> The ride down the line was well worth the stress...

The other time that stands out was during scuba certification class. The first giant stride into the deep end. ~50 lbs of gear on, hindbrain screaming "you are going to sink straight to the bottom and die!" <img src="/images/graemlins/shocked.gif" alt="" /> Once again, trust the gear, trust the instructors...

Like they say, bravery is not the lack of fear, but not letting fear control you.