Borrering is a Viking-age ring fortress located on the island of Zealand in Denmark, believed to have been constructed in the late 10th century under King Harald Bluetooth. Archaeologists have identified it as one of approximately five such ring fortresses spread across Denmark, each engineered to a remarkable 470-foot diameter circle with four gates precisely aligned with the compass points. The fortress would originally have been built from the earth itself, with timber beams shoring up the bank to form a defensive perimeter enclosing a series of Viking longhouses inside. A skeletal frame erected by archaeologists now marks the site's one-time footprint, giving visitors a sense of its original scale. Gates visited Borrering in 2016 because it was a recently-discovered fortress — potentially linked to Harald Bluetooth's campaign to consolidate Viking power across Denmark — and excavations were still actively underway when cameras rolled.
Borrering fortress believed constructed under King Harald Bluetooth as part of a coordinated network of ring fortresses across Denmark
Harald Bluetooth rules a unified Denmark for approximately 30 years, reportedly commissioning at least five ring fortresses including Borrering
Borrering rediscovered and identified by archaeologists; excavations begin under chief archaeologist Nanna Holm
Gates visits the active dig site for Expedition Unknown S04E01, "Viking Secrets"
Chief archaeologist Nanna Holm, who appears on camera during Gates' visit, confirmed that Borrering is one of five ring fortresses identified so far across Denmark — all believed to have been built at roughly the same time under King Harald Bluetooth in the late 10th century. Holm told Gates that the fortress was originally constructed from the earth itself, shored up with timber, and could be completed in less than a year — a remarkable feat that implied significant centralized organization and manpower.
In mainstream archaeology, the ring fortresses (sometimes called Trelleborg-type fortresses after the best-known example) are considered some of the most compelling evidence of Harald Bluetooth's administrative and military ambition. Their shared 470-foot diameter, compass-aligned gates, and internal longhouse layouts suggest these were not ad hoc constructions but the product of a unified engineering plan — an early indication of the kind of state-level organization that would define Scandinavian kingdoms in the centuries that followed.
Harald Bluetooth also holds a notable place in the history of technology, albeit an unexpected one: the modern Bluetooth wireless standard was named after him as a tribute to his unifying legacy, and the Bluetooth logo itself incorporates the runic initials for "Harald" and "Bluetooth." Nanna Holm pointed this out to Gates on camera, prompting his reaction: "So every time we say Bluetooth, we're really talking about a Viking king."
What Gates' episode contributed was a rare on-camera look at an active, freshly-discovered excavation — Borrering had only recently been rediscovered when filming took place, and the central area of the fortress had not yet been systematically excavated. The episode does not claim to have definitively identified the longhouse structures or confirmed any headquarters connection to Bluetooth, but it does document early physical evidence — including a wagon wheel fragment and postholes — that archaeologists hoped would shed light on daily life inside one of Viking-age Denmark's most ambitious military projects.
The modern Bluetooth wireless standard takes its name from King Harald Bluetooth, and its logo is a combination of the runic initials for 'H' and 'B' — Harald Bluetooth.
Borrering is believed to have been constructed in less than a year, according to archaeologist Nanna Holm, suggesting Harald Bluetooth could mobilize an extraordinarily large and organized labor force.
All five known Danish ring fortresses share the same 470-foot diameter and the same layout of four compass-aligned gates, pointing to a single coordinated engineering plan rather than independent local construction.
Borrering was only recently rediscovered before Gates' 2016 visit, and the central area of the fortress — where longhouse remains and potentially Bluetooth's own headquarters might lie — had not yet been excavated at the time of filming.
Borrering is located on the island of Zealand in Denmark, and the site is generally considered accessible to visitors, though it remains an active or recently active archaeological area — conditions on the ground may vary. The skeletal frame erected by archaeologists to mark the fortress's original footprint is the primary visual feature visitors can expect to see. It is advisable to check with local Danish heritage authorities or the nearest regional museum for current access information before visiting.
The site is located on Zealand; Copenhagen, Denmark's capital, is the nearest major city, approximately 30–40 km to the northeast depending on the precise access route.
Late spring through early autumn (May–September) offers the most comfortable weather for an outdoor archaeological site in Denmark. Summers are mild and relatively long, making it easier to appreciate the landscape context of the fortress.
Scandinavia
Scandinavia appears in the Expedition Unknown database as a broader regional investigation context, closely connected to Viking-age history and culture explored at Borrering.
Scotland
Scotland shares deep Norse cultural and historical ties through Viking-age settlement and raiding, making it a natural companion to investigations of Viking military infrastructure.
Lake Champlain
Lake Champlain has been associated with legendary historical mysteries investigated by Gates, reflecting the same pattern of using archaeology and expert consultation to probe evidence from a distant but connected past.