High water mark: the highest reach of the water at high tide.

UNESCO: L O S S A R Y

Late this morning was high tide along much of the coast, and the highest water levels of Isaac are likely being experienced at many locations.

FORBES: Hurricane Isaac and New Orleans: It's All About Water

The High-Altitude Water Cherenkov Observatory or Hawc, high on a Mexican plain, now holds the record for the highest-energy light it can capture.

BBC: Hawc gamma-ray telescope captures its first image