Helcar was the name of a great tower.
Helcar (also spelt Helkar) was raised by the Valar (some traditions say by Melkor) to house the northern of the Two Lamps, Illuin. It was placed in the north of Middle-earth, and together with the soutern lamp Olmar lit up all of Arda.
Helcar was destroyed by Melkor, and collapsed. This ruined a great part of Middle-earth, and waters (probably from Belegaer and the eastern ocean) flowed in. Later here was formed the inland Sea of Helcar.
In some traditions it was said Helcar was made by Melkor out of ice, a substance not yet known to the Valar. The heat of Illuin then melted Helcar, and the meltwater of this tower formed the Sea of Helcar.