The Arctic Circle was a line of latitude north of the equator on Earth, which marked the boundary between the North Pole, or Arctic Pole, and regions south of the circle. It passed through the continents of Asia, Europe, and North America.
In the late 17th century, the Arctic Circle, located under the Kara Sea, was labeled on a map of Earth. The label for this latitude was in Latin - "Circulus Arcticus". (Star Trek: Enterprise opening credits)
In 2063, the USS Enterprise-E pursued a Borg sphere to Earth from the 24th century through a Borg-created temporal vortex. The Borg had arrived to destroy the Phoenix, Zefram Cochrane's historic first warp-powered vessel. The Enterprise easily destroyed the sphere, though the Borg crew escaped. (Star Trek: First Contact) Unknown to the Enterprise crew, much of the debris from the Borg ship survived and landed in the Arctic Circle. (ENT: "Regeneration")
Nearly a hundred years later, an Earth Sciences Institute team discovered the debris field and recovered two Borg drones from beneath a glacier. The drones thawed and assimilated the researchers and their ship. (ENT: "Regeneration")
Background info Edit
The 17th century map, known as "Harmonia macrocosmica seu atlas universalis et novus, totius universi creati cosmographiam generalem, et novam exhibens", was created by Andreas Cellarius. A scan of the original can be found here.
According to a 21st century-era political map of Earth, seen in the remastered "The Cage", the Arctic Circle was 66° 33ʹ north of the equator. This was a Central Intelligence Agency map published either in April 2007 or September 2008. The difference, which was not noticeable at the resolution of the image from the episode, was the addition of Kosovo, which gained its independence between the releases. The label "Arctic Circle" was illegible to the viewer.
It was possible that the Arctic Circle was on two 20th century-era political maps of Earth used in the original "The Cage". However, due to the poor quality of the images and the inability to identity and access the source material for verification, this was far from certain.