Why Does Water Reflect Images Like This?

The salt flat covers more than 10,000 square kilometers and yet its altitude varies by no more than a few feet across the entire plane. When nearby lakes overflow during the rainy season, the salt flat will fill up with a very shallow sea of water. In some places, it appears to be only inches deep. A very still puddle like that can essentially create a gigantic mirror reflecting the sky all the way to the horizon.