 Why was the cave the only place that was not visited?
EVERYTHING IT HAD WAS IN THE GARDEN BY R. A. LAFFERTY But they were disappointed that they found nothing for several hours as they hovered high over the rotation. As though there were but one city, if that is its form. On Earth it had read Positive on a number of crack-pots, including Waxey Sax, a jazz tootler who could not even read music. The Little Probe hovered stationary in the light and the moon-town came up under the dawn. And those two ... why, they could well be Earth-people. There was a man and a woman, and they were clothed either in very bright garments or in no garments at all, but only in a very bright light. I don't understand that light that surrounds them.
 The serpent lives there and the crew was told that he was cranky.
 The cave was only a reflective illusion from the bright light.  The crew ran out of time but planned to examine it upon their next arrival  Adam, or Ha-Adamah, told the crew that it was much to dangerous as there were evil creatures living inside.  The serpent lives there and the crew was told that he was cranky.