This is not some strange specimen under a microscope, but a composite image of two satellite photographs from the NASA Earth Observatory web site showing the high and low water level in Lake Mead in 1984 (high) and 2016 (low). What you are seeing is the difference between those two images—black means there was no change. The bright fringe around the lake is how far the water has receded. It also shows the development of Las Vegas to the west of the lake. The dark center of the city is where it was already developed in 1984. Most of this water loss and development has happened since 2000. Here are the original images:
Lake Mead and Las Vegas in 1984.
Lake Mead and Las Vegas in 2016.
Click on the images for a larger view. For more on this story and to see higher resolution images, visit the NASA Earth Observatory.