Assimilated Arctic One at warp.
Enterprise attempting to destroy Arctic One.
Arctic One was a 22nd century transport used by a group of Earth Sciences Institute personnel when they discovered the remains of an unknown craft in Earth's Arctic Circle in 2153.
Unbeknownst to the scientists, the craft they discovered was of Borg origin and were assimilated for their troubles. The new drones then attempted to use Arctic One to travel towards the Delta Quadrant to contact their Collective.
En route, the ship was destroyed by Enterprise, but it was able to send a homing signal to the Borg, alerting them of Earth's location. (ENT: "Regeneration")
- The reason the ships mass keeps growing isn't directly explained in the episode, but it was the writers intentions that their ship would eventually round into a sphere, to imply that the Borg don't have traditional ship yards, but take parts of ships they assimilate and build them into their ships. This information did not make it into the episode in it's entirety though.