Around 25,000 years ago, hunter-gatherers used the bones from 60 mammoths to build a large circular structure in Russia.

And no one knows why.

Researchers have excavated the site in an attempt to understand it, but they don't know why the structure was built, according to a new study.

This isn't the first "mammoth house" to be found in Russia, but it is the oldest and largest, measuring 41 feet across.

In the 1960s and '70s, researchers found similar, smaller buildings at the site, which they dubbed Kostenki 11. It's 310 miles south of Moscow and now home to a museum, the State Archaeological Museum-Reserve Kostenki.

In 2014, researchers found evidence of this structure at the site and began excavation in 2015, which took three years. A study detailing their findings published this week in the journal Antiquity.

These mammoth bone structures, dating to the Ice Age, have been found across Eastern Europe. But until now, the oldest ones found were dated to 22,000 years ago.

Based on previous discoveries, researchers believe they were constructed by Palaeolithic people to serve as houses, providing refuge during harsh winters. Ice Age winters likely had lows reaching negative four degrees Fahrenheit.

And constructing something this massive out of hundreds of mammoth bones would have taken time. It's surprising, considering that populations of hunter-gatherers never spent much time in one location.

"Mammoth bones are very heavy, and building the circular structure represents a huge investment of time and energy by the humans that built this," said Alexander Pryor, lead study author and Palaeolithic archaeologist at the University of Exeter.

The bones formed a continuous circle, with no obvious entrance, according to the study. So far, the researchers have identified 51 mammoth mandibles and 64 skulls, the researchers said.

Inside the circle, the researchers also found the first evidence that wood was burned inside it. But overall, there are no signs of long-term habitation inside the structure.

The researchers believe that it didn't act as a wintertime refuge, which has them rethinking the purpose of these massive, time-consuming structures.

"It clearly meant something to them, and there was very likely a ritual element to it, even if the structure ultimately had some sort of practical purpose too," Pryor said.

Investigating a mammoth house

As the first "mammoth house" found in more than 40 years at Kostenki, the researchers took advantage of new techniques to investigate the site that weren't available for the structures previously found nearby.

This included flotation, something fairly new when the other structures were excavated. Archaeologists use flotation to separate material from soil, using water and sieves. This allows for the discovery of tiny fragments, which can provide the minute details of a larger story.

Flotation helped the archaeologists find evidence of a possible food source for the hunter-gatherers at the site -- besides the mammoths.

"We found pieces of soft plant tissue typically found in edible roots or tubers, hinting at a plant food component in peoples' diet," Pryor said. "These finds are important because they illustrate how our human ancestors adapted to survive the harsh environments of the last Ice Age by making use of the resources they found around them."

It was also flotation that revealed the tiny bits of charred wood, the first evidence that humans burned wood fires in the mammoth structure. And it suggests that wood was still a resource used by hunter-gatherers during the Ice Age.

Other areas in Northern Europe showed signs of abandonment at the time, and suggest that trees were a rare and precious resource. This may hint at why Kostenki was chosen.

"This demonstrates that trees were still present in the landscape, despite the harsh climatic conditions of the time," Pryor said.

The discovery also inspires more questions for the researchers because they believed hunter-gatherers built these structures out of mammoth bones because wood was unavailable. This makes the choice of mammoth bones deliberate. But why?

"It's not yet clear whether the bones are from mammoths recently hunted and killed by humans or if they were scavenged from carcases of animals that died of natural causes," Pryor said.

The researchers plan to investigate the site further, in hopes of determining the purpose of the structure.

"Kostenki 11 also seems too large to be a dwelling," Pryor said. "It is difficult to imagine how an area this large could be roofed over using materials available in this environment."

One suggestion by the researchers is that the site's purpose was food storage, which helped them survive the Ice Age and its devastating winters.