Dante Gabriel Rossetti: Persephone, 1874
I have always known about Lethe as the river (or pool) of Forgetting, but the Pool of Memory (i.e. of Mnemosyne, mother of the muses) has remained unknown to me until now, when I found it mentioned in a poem by Ed Hirsch:
~ Edward Hirsch, from “The Hades Sonnets,” Lay Back the Darkness
The pool of memory is a late, post-Homeric development, an "addition" to the House of Hades that’s related to the Orphic mysteries.