The Odyssey: Nekyia

In book 11 of the Odyssey, the hero Odysseus (Ulysses) makes a different kind of journey in order to find the only person who can help him find his way back to the island of Ithaca, his home. It happened that person, the prophet Teiresias, was dead so he had to go to the land of Lord Hades, king of the Underworld where the Dead dwelled according to the Old Greeks. According to this story, dying was not the only way to descend to the Land of the Dead. some Greek heroes descended  while they were very much alive.  The Greek underworld could apparently be accessed in different points. Orpheus for instance just descended to it by only walking and so did Herakles. In this case, Odysseus sails there following the witch Circe's indications, to the very edge of the river Ocean.

Once he gets there Odysseus makes a sacrifice killing a lamb and offering the blood to the dead, a rite which the Greek called Nekyia. One by one people who had met Odysseus approached him attracted by the blood. They drank the crimson liquid and remembered their past existence. Thus,  Odysseus gets to see old friends and colleagues, his mother, also Teiresias, and even other heroes from the past. Odysseus managed to experience what very few men in Greek literature had: descending to the Underworld and returning alive.
The idea of an existence beyond this one is as you can see quite old  as it is the notion of the spirits of the dead existing in some underground realm. Later on Christians would come to adopt a similar belief only that their underworld would be called Hell. In the Greek underworld both those who had been good and those who had been naughty went to the same place. The only difference is that what Christians call Heaven was also a section of the Underworld and so the Wicked occupied the rest of the land of Hades (subdivided in sections by five rivers), but the ones who deserved better were sent to a place of eternal happiness and sunshine. It was called the Elysian Fields, the Greek version of Heaven.