This is the fourth of five edited extracts to be posted this week from my new book, Prehistoric Belief: Shamans, Trance and the Afterlife. After the Neolithic yesterday, we now move into the Bronze Age, when people found novel ways to journey to the otherworld.

Having shrugged off most of our clothes, we crawl through the low doorway into a small cramped chamber. It is too dark to make out the form of the walls but it is clear that any gaps have been plugged with animal hides. Some have obviously not been cleaned very thoroughly and, in the confined space the smell makes us gag. As we crawl across the bare earth, we are careful not to bump into the water trough that takes up the middle of the space. We huddle near the back and wait whilst the others crawl in; there is an old woman, two men, and a girl. They arrange themselves around the trough and the old woman starts to chant. Almost immediately, a wooden paddle is thrust through the doorway, with several glowing rocks on its end. We noticed a small fire being lit as we entered the tent and now we know what it is being used for. The rocks are dropped into the tough and then more and more follow until the water starts to boil. The flap of skin covering the entranceway is then swung shut. It is now as black as night. Waves of hot steam begin to billow against our faces and we draw the thick, hot air into our lungs. More hot stones are added to the trough whenever the boiling subsides until the atmosphere becomes almost unbearable. Still, the old woman continues her slow, lilting wail. We are now fighting for every breath and can feel the stifling air pressing down on our bodies. You lie flat, hoping to take comfort from the cool of the earth. Our heads pound, and still the wailing sound continues. More hot stones are added to the water and we feel as if we might pass out from the heat. Then, quite suddenly, I feel a calmness. Shapes form in the darkness, and then a tunnel. I know it is the entrance to the otherworld and I wonder if you see it too.

Anyone familiar with the Native American sweat lodge might think that is what we have just experienced, since an almost identical practice forms an important part of many Native spiritual traditions, such as the Lakota inipi. However, the sweat lodge we visited did not exist in America but in Europe during the late Bronze Age.

Piles of burnt stones, often associated with a small trough and makeshift structure, were widespread across much of Europe at this time, often occurring next to flowing water. Hot stones were probably added to water in the trough and experiments have shown that this will rapidly cause it to boil. At first, it was assumed that people used the boiling water to cook joints of meat and, where burnt mounds occur on settlement sites, or have food residue around them, this may have been what they were used for.

At many burnt mounds sites, however, there is a dearth of food remains and, although the technology was identical, the water was clearly being used for something very different. Boiling water produces copious stream and, if trapped within the small structures that appear next to the mounds of burnt stone, would form a rudimentary sauna. While it may be that people in the Bronze Age had high standards of personal hygiene, the remote location of many of these sites makes another explanation more likely. Lengthy exposure to steam (coupled with the resulting abnormal body temperatures) is sufficient to induce trance. The Nenet shamans of Siberia use precisely this method to access the otherworld and it is likely that people in the Bronze Age were doing the same.

One reason for this may have been to allow people to visit the dead and in Ireland, many burnt mounds were located close to burial cemeteries. Similarly, in Scandinavia, some of the burnt mounds may have doubled-up as cremation pyres. One mound, located on the island of Pryssgården, had a spiral design of stones fashioned at its base, mirroring the tunnel that gives access to the otherworld.

At Bargeroosteveld, in the Netherlands, a curious structure within an area of raised bog may have been used for similar purposes. It comprises a small open building surrounded by a ring of stones, with horned ends to its roof-beams. Three bronze hoards buried around it may have been offerings for the spirits and at Tauberbischofsheim-Hochhausen, in Germany, a wooden post served as a focus for more metal hoards, concentrating, most appropriately, on spiral-decorated ornaments. Like the stone design at Pryssgården, it seems that spiral designs marked portals between the worlds.

Some burnt mounds in Cambridgeshire, in eastern England, formed part of larger ceremonial complexes and these could sometimes include upside-down trees. This is very similar to another nearby site, named with no more accuracy than its famous namesake: Seahenge. Fifty five split oak posts formed a circle, bark-side outwards, with a huge, upside-down, oak tree at its centre. The roots reached at least a metre into the air. Although it was made in the early Bronze Age (it can be dated exactly by its timbers to 2049 BC), there are signs that it was still in use in later periods. A track-way dating to the middle Bronze Age crossed a creek nearby and, with an absence of houses in the vicinity, it is likely that people were coming here to visit the monument. The immediate area, now almost inundated by the sea, would have been wet, marshy ground, a liminal location that matched the structure at Bargeroosteveld.

The upside-down tree which formed the main focus of Seahenge, has parallels in other Eurasian traditions. The Sámi of Lapland, for example, placed upside-down trees into the earth either to mark the position of offerings to the spirits or as offerings themselves. For the Evenks of Siberia, it was not offerings but the shaman who passed through the portal, and upside-down trees were arranged to form a route along which the shaman journeyed to the otherworld. It was likely that Seahenge was used in the same way, perhaps with people lying upon the inverted bole of the tree as they let their spirit be pulled to the otherworld. Intriguingly, near to Seahenge is a similar circle of posts with two large timbers at the centre. Hollows on the upper sides of both timbers may have been designed to hold a coffin or other platform and perhaps this was where the dead were once laid out, allowing their souls an easy path to the afterlife.