Fathomlethes are tiny pearls, described to look like blue-green caviar, which can be found in rare river oysters. They were kept in a bottle and removed with tiny tongs. As mentioned on page 222 of Neverseen, once you eat one, you have about 15 minutes before it puts you to sleep. They are said to give you crazy dreams but can also help you access long-term memories. Keefe ate one in book 4 to help him try to find clues in his memories of Lady Gisela's plans and hints of her being in the Neverseen. In book 7, Keefe tried to convince Elwin and Tiergan to give him some so he could recover more memories, but Elwin refused.