If the soul is immortal, Socrates points out, our actions in this life will have consequences that will last forever. As a result, it is of the utmost importance to care for one's soul and live properly. Socrates recounts a myth of the afterlife to illustrate this point.
After death, we are all brought to a place of judgment, from whence we are led by a guide to the other world. Those who have lived an evil life, attached to the flesh, will have to be forcibly dragged away from this life to their proper place, whereas those who have done good will happily be led to their destination.
Socrates explains his belief that the earth is spherical, in the middle of the heavens, and in perfect equilibrium. The earth is also incredibly vast, and the known world is but one of many hollow places in the earth where water and mist and air have gathered. The true surface of the earth, far above us, is a pure ether. We are like creatures living at the bottom of the sea who assume they are on the surface of the earth and that the sea is the sky. If they were brought out into the open air, they would see how much more pure and beautiful the real world is than their murky, corroded aquatic world. Similarly, we are wrong to think that we are on the true surface of the earth or that the sky is the true heavens: these are just the dregs of a far more pure and beautiful earth of which we are mostly unaware.
The true earth, viewed from above, is a sight to behold. It is marked by bright colors, some different from any colors we know. The plants are also pure and beautiful, and the mountains are smooth and made entirely out of rubies, emeralds, and other precious stones, as well as stones more precious than any of which we know. Gold, silver, and other precious metals are also to be found everywhere.
The people who live here are surrounded by air and ether, and are superior in every way: their senses are more acute and their intellect sharper. Their temples are inhabited by the gods themselves, whom they speak to directly. They also see the sun and moon and stars as they really are, possessing a true understanding of the heavens.
All the hollow regions of the earth are connected by great subterranean rivers of water, fire, and mud, which flow between the several regions. One of the cavities in the earth is so large and so deep that it pierces right through to the other side of the earth. This cavity, often referred to as Tartarus, is where all the rivers flow together and where they flow forth from again. The greatest of these rivers is Oceanus, the ocean that surrounds the world The great underworld rivers of Acheron, Pyriphlegethon, and Cocytus also flow to and from Tartarus.
When people die, those who lived a neutral life set out for Acheron, and spend a certain period of time in the underworld, where they are punished for their sins and rewarded for their good deeds, and then are returned to the earth once more. Those who have been irredeemably wicked are hurled into Tartarus, never to return. Those who have been good, however, ascend to the true surface of the earth, and those who have completely purified themselves through philosophy will live without a body altogether, and will reach places indescribably more beautiful even than the true surface of the earth.
Obviously, this myth is not intended as a literal account of what Plato thinks the earth or the afterlife is like. Instead, it is a way of summarizing and contextualizing many of the themes in the dialogue in a very attractive and readable way. (We find similar myths at the end of the Gorgias and the Republic.)
The main focus of the myth lies in the contrast between the world as we know it and the true surface of the earth, which is in every way more pure and beautiful than what we know. The analogy here, quite clearly, is to the contrast between the sensible, immanent world that we know, and the invisible, transcendent world of Forms that is being posited. Just as what we see and touch are pale reflections of the Forms in which sensible objects participate, the world we inhabit is a pale reflection of the true surface of the earth. The air and water and mist that surround us are but the dregs that seep down from this more perfect world.
According to the myth, people who live on the true surface of the world can speak directly with the gods and can see the sun and moon as they truly are. By contrast, the ancient Greeks could only communicate with their gods through oracles, and a proper understanding of the heavens was considered the highest and most difficult task of physics. The people who live on the true surface of the earth are the model philosophers who, through an understanding of Forms, can see things for what they really are. In the Republic, Plato gives us the famous analogy of the cave where ordinary people are seeing mere shadows of puppets representing the real world, while philosophers, through an understanding of the Forms, can leave the cave and see things as they really are.
Plato's myth, and his account of the afterlife, have also had a profound effect on Christianity. In the traditional Greek account, all souls suffered the same fate after death, descending to Hades, where they remained as shades for eternity. Plato's account is an early version of the Christian ideal, in which our fate after death is determined by our virtue in this life. The evil are swept into an eternity in Tartarus in the middle of the earth, while the good ascend to a pure and perfect world. The only difference is that the good for Plato are not those who possess Christian virtues, but those who possess a knowledge of Forms, and thus a proper understanding of things. Plato and Socrates both are of the opinion that knowledge and goodness are one and the same.