The site was never hidden. In 1875 the antiquarian Sophus Müller noted a bank east of Gl. Lellingegaard called Borgerring and remarked on its position beside the stream, offering no date. A US Air Force aerial survey in 1954 caught it as a faint circular shadow in the crop. In November 1970, echo-photography from Air Base Karup produced an image in which warrant officer Valdemar Ryhl recognised a ring, and the possibility of a trelleborg near Lellinge was raised in print. Two years earlier, when the National Museum drew up a protection order for the bank, Hans Stiesdal had written a note on it by hand: the declaration should be discussed more carefully, he thought, because "the presumed ancient monument seems somewhat dubious." He turned out to be wrong, but he was not being unreasonable.

In 1971 and 1972 the Danish National Museum sent Thorkild Ramskou to settle the question. Everyone expected a Viking fortress. What the trenches produced was settlement material and middens from the Roman Iron Age, no trace of later occupation, and — because the dating techniques of the day could not do better — no date at all for the embankment itself. The expectation was not satisfied, and the site slid back into obscurity. When Harald Andersen surveyed Denmark's prehistoric fortifications for the magazine Skalk in 1992, Borrering appeared in his list as a circular earthwork about 140 metres across, ploughed flat, with finds from the early Roman period. A circular fortress outside Køge, pottery-dated to Roman times: that was the state of knowledge for twenty more years.

Sindbæk's insight was geographical, but the confirmation was geophysical. In August 2013 Helen Goodchild of the University of York ran a geomagnetic survey across the field and produced a detailed shadow-image of the buried structure — a ring, complete, with the interruptions where gates should be. Excavation followed in 2014, run by Danmarks Borgcenter and Aarhus University. The northern and eastern gates turned up precisely where a trelleborg plan predicts them. There was no defensive moat, but the Ellebæk stream runs past the western side and a small lake lay to the north-east. One detail is oddly human: the gates are not aligned exactly north, east, south and west but rotated about eleven degrees, most likely a deliberate correction so the fortress would not look lopsided in a landscape that is not itself level.

This is where the story stops being tidy. Radiocarbon results pointed to the 900s, consistent with the other trelleborgs, which cluster around 980. In June 2017 a piece of timber roughly a metre long was recovered from the valley south of the fortress and dated by tree rings to a tree felled after 966 — a good date, but attached to a utilitarian object rather than to the rampart, so it fixes activity nearby rather than the building of the fort. Only the two gates and a stretch of the southern rampart have been excavated, and too few post-holes have been found inside to show what stood there. Harald Bluetooth is presumed to have built the other four, which makes him the obvious candidate here; presumption is not proof, and the honest summary remains that his involvement is likely but not demonstrated. Burnt gates at both entrances suggest violence — nineteen arrows were found shot into Trelleborg at Slagelse — though fire has other causes.

When preliminary results were published in September 2014, the Chapter of Vallø, which owns the land, wanted the site called Vallø Borgring. Objections were loud and immediate. A Facebook group appeared on 8 September 2014 campaigning for Lellinge Ringborg instead. On 10 October the Danish Cultural Heritage Agency settled on plain Borgring, on the grounds that everyone would know what was meant — a practical decision rather than a historical one, since nineteenth-century maps put the name Borgring on a small wood some 300 metres to the west. The older written form, Borrering, goes back to a land register of 1682, and the archives hold Borre Ring, Borgering, Borgerring, Boring and Borgringen besides. In 2023 the fortress joined Trelleborg, Fyrkat, Aggersborg and Nonnebakken on UNESCO's World Heritage List as the Viking-Age Ring Fortresses, testimony to the reach of the Jelling dynasty. It is a considerable honour for a mound that, sixty years ago, a museum officer thought looked dubious.