Apparently it takes the strength of the full moon both to do this and to summon the Dweller in the Pool.

We will first try a concentration of the rays of the waning moon upon these circles to see whether that will open the rock.

If it does we will be able to investigate the Pool without interruption from--from--what emanates.
"Look, here on the chart are their locations.

I have made this in duplicate for you in the event--of something happening--to me.

And if I lose--you'll come after us, Goodwin, with help--won't you ?" And again I promised.
A little later he complained of increasing sleepiness.
"But it's just weariness," he said.