The concept and nature of the underworld (𒆳 kur) seems, at a surface level, to be one of the most difficult concepts to confront in the Sumerian religion. An internet search for "Sumerian afterlife" yields results on how the underworld is a universally cold and miserable place, with clay to eat and mud to drink, which is the ultimate fate of all humans.
This is a misunderstanding and comes from a conflation of Sumerian source material with that of later Mesopotamian civilisations. The concept of the dismal, grey afterlife comes from a Neo-Assyrian (1st millennium BCE) version of the Descent of Inana, written in Akkadian:
"The daughter of Su'en directed her ear to the house of darkness, the place Irkalla, from which those who enter may never leave. Those who enter are denied light. They feed on dust and clay. They see no light, they sit in darkness, clothed like birds in garments of feathers."
Compare the ancient Sumerian version of the same story, which states simply:
"Inana abandoned heaven and earth and went down to the underworld."
Why should the later version be so drastically different? We can only speculate, but we should remember that grandiose and embellished language was a Semitic literary device (compare the apocalyptic text of the Book of Revelation). As the original literature makes clear, the Sumerians did not conceive of the afterlife as being universally awful.
What, then, did the afterlife look like to the Sumerians?
Firstly, there was no single, universal fate. The depictions of the afterlife that we do have describe people ending up in different situations after death. There was a judgment after death, perhaps rendered by the council of gods known as the anunnaki. The Sumerians believed that the king Ur-Namma joined the pantheon on his death, and it is described that:
"At the command of Ereškigal [...] he [ie, Ur-Namma] will pass the judgements of the nether world concerning those men who fell to a weapon."
Similarly, the moon god Nanna is described as passing verdicts in the underworld on the "day of the disappearance of the moon"; the day of the new moon, the last day of the month of the Mesopotamian calendar.
What this judgement constituted seems to reflect how a person upheld the moral order of the gods in life. The tale of Gilgameš, Enkidu and the nether world tells of how the moral and upstanding are able to enter the palace of the underworld, with those especially righteous sitting as companions of the gods and listening to judgments. Translations vary as to the fate of the unrighteous, but those who lie to the gods in prayer, and those who extol their own virtues, are consigned to wander the afterlife as a mere shade of their former selves.
The story of Gilgameš and Enkidu also reveals that there is no Sumerian "original sin"; stillborn children go to the underworld where they are seated at a table laden with gold and silver, honey and ghee - indicators of luxury in Sumer.
It also tells that the worst fate is to ascend into the heavens, because this is the dwelling of the gods, and not a place for humans. This is seen as a fate worse than even the meagerest existence in the afterlife, and is reserved for those whose bodies are cremated.
A key component of the Sumerian afterlife is that earthly goods can be taken with you. The physical world reflects the spiritual world, and physical goods that a person is buried with will appear with them in spiritual form in the afterlife. What's more, offerings such as food and drink can be made to the deceased in the same way as could be made to the gods, and such offerings were believed to nourish the deceased.
Because of this, it is important to be buried with things that one may need to live a comfortable existence in the afterlife. This includes items of both physical and spiritual importance.
Offerings could be made to the dead, and this was in fact believed to be necessary to nourish the deceased in the afterlife. Such offerings would take similar ritual form to those made to deities, and would be generally carried out by the family of the deceased.
It was also abundantly possible for gods to provide blessings for their followers in the afterlife. The idea that the gods have no power to interfere with the workings of the underworld is another later Assyrian interpretation; in Sumerian stories, gods and heroes frequently come and go from the underworld without any trouble. A prayer to the goddess of healing, Nintinuga, beseeches:
"When I die, may [Nintinuga] provide me with water in the underworld."
Clearly the Sumerians thought it possible for the gods to grant boons to the deceased, otherwise there'd have been no point in praying for it! The king Ur-Namma is also described as holding a feast for the denizens of the underworld upon his arrival there, a far cry from the dismal existence the Sumerian underworld is often portrayed to be.
There is, furthermore, evidence that the Sumerians did not consider death to be the end of the line. An early Babylonian version of the Descent of Inana foretells a time when the water of life will be brought up from the underworld, which appears to be a time when divine decrees will be carried out that bring the world into a state that it was meant to be, a state where human society is a true reflection of the order of the cosmos as intended by the gods.
Perhaps the most compelling evidence against the hypothesis of the bleak afterlife is a proverb from the Sumerians themselves:
"Water is poured, and drunk by the ground. In the underworld, the most honoured place, it is a libation."
Ultimately, the Sumerians believed that death, and the afterlife that was to follow, was a part of the human experience, and was decreed by the gods to be that way. It is intended that we focus our attention on the life that we have on the Earth, and to recognise death as the great universal leveller that it is.