In 2017, the town of Tvååker, Sweden, started a city planning project in hopes of installing new water pipes and a roundabout.
Archaeologists with the government organization Arkeologerna -- part of Sweden's National Historical Museums -- were called to the site to conduct routine excavations and check for any historical artifacts, the organization said in an Oct. 15 news release.
The site has been excavated in stages, archaeologists said, and they discovered a few items dating to the Stone Age, including flint and a furnace.
The Stone Age ended in Sweden around 1800 B.C., according to Scandinavian Archaeology, but as the team began digging section by section through the site, they started to find burials that were not nearly as old.
Petra Nordin, manager of the project, said in the first year they discovered five graves with human and dog bones, and layers of fire, according to the release.
The burials were dated to between 800 and 1050 A.D. -- the Viking era.
Before long, more and more burials were found, and other Viking-age artifacts emerged from the dirt, archaeologists said.
It was a massive burial ground, the organization said, and in the years since the excavations began, archaeologists have only been able to reach 6% of the total site.
In just that small portion, archaeologists have found nearly 140 burials, multiple large shipwrecks and dozens of artifacts.
The site is strategically located on a ridge that straddles two transport routes, the organization said. On one side, the Tvååkersån river flows towards the North Sea. On the other side, a country road crosses the burial ground to reach towns mentioned in old historical sources, archaeologists said.
However, in the centuries since the Vikings used the site, it has been plowed for agriculture, and archaeologists said any structures that once stood on the ridge were torn apart and graves were broken and dispersed.
That hasn't stopped the team from making large discoveries.
In one section of the site, archaeologists uncovered a more than 160-foot-long shipwreck, as well as two other shipwrecks of unknown size and a ship-shaped mound. The organization believes much of the burial site may still be under modern construction.
Many of the burials had both human and dog remains, as dogs were treated as companions and regularly joined their owners in death before the remains were burned, the organization said.
Other graves contained cow bones, suggesting they were placed on top of the burned person as a food sacrifice and burned, Nordin said.
In other locations, archaeologists didn't realize a pit is a grave until they use a metal detector to find artifacts that were buried with the bodies.
For example, multiple types of buckles, ceramics and an Arab silver coin have all been discovered. The coin dates to between 795 and 806 A.D., which matches the approximate age of the oldest grave discovered so far, the organization said.
Despite years of discoveries at the site, archaeologists are still wondering where the people who were buried there actually lived, the organization said.
The team believes there may have been an older Viking Age village where Tvååker stands now, or that a trading post may have been built near the harbor to the west. This question remains a mystery as excavations continue, the organization said.
Tvååker is in southwestern Sweden, across an inlet of the North Sea from Denmark.