The Ancient Fountain is a non-member den item. It was released on May 24, 2018, and can be obtained by completing The Mystery Below in hard mode. It is in the bottom left chest.
The Ancient Fountain is made out of stones in a shape similar to the pillars in the underground city. Each side corner has a blue outline and a crystal on it and the fountain has two light blue waves and two light blue dots. Water can be seen flowing from the top and into the water while water shoots out from the sides. This item comes in one variety.