  We will discuss selected physical effects of spacetime geometry fluctuations,
especially the operational signatures of geometry fluctuations and their
effects on black hole horizons. The operational signatures which we discuss
involve the effects of the fluctuations on images, and include luminosity
variations, spectral line broadening and angular blurring. Our main interest
will be in black hole horizon fluctuations, especially horizon fluctuations
which have been enhanced above the vacuum level by gravitons or matter in
squeezed states. We investigate whether these fluctuations can alter the
thermal character of a black hole. We find that this thermal character is
remarkably robust, and that Hawking's original derivation using transplanckian
modes does not seem to be sensitive even to enhanced horizon fluctuations.
