The left is a detail of The Delphic Sibyl, 1509, that is at the Sistine Chapel. I can look for minutes at her and can't get enough.
"They (the sibyls) were women of ancient times supposed to have supernatural gifts of foretelling the future. They devoted themselves to solitude and meditation, and sometimes lived apart in caves or grottoes. Sometimes they were connected with temples, and delivered what were supposed to be the messages of the gods to the worshippers. These messages were called oracles, and were greatly revered by the people who consulted the gods.
Some of the sibyls' words of wisdom were committed to writing and passed down to following generations. Though they lived in heathen countries, the tradition ran that they prophesied the advent of Christ. There is a passage in one of Virgil's eclogues (the fourth) upon which the supposition is based. Early in the Christian era, when men were spreading the new faith, they made much of these sibylline prophecies to add weight to their teachings." (keep reading at the site).
About the Sibyl:
"The Delphic sibyl, or pythia, of Michelangelo's picture, has the splendid stature of an Amazon. Her head is draped with a sort of Greek turban, beneath which her hair escapes in flying curls. Her face and expression show her at once to be unlike an ordinary woman. She has the look of a startled fawn, which has suddenly heard the call of a distant voice. She turns her head in the attitude of one listening. She looks far away with eyes that see visions, but what those visions are none can guess. There are other pictures of the same sibyl carrying a crown of thorns, showing that she predicted the sufferings of Christ. Perhaps this is the meaning of the sorrowful expression in these wide eyes."