Map of Elostirion (slightly conjectural)²

The tallest of the three Elf-towers that stood on Emyn Beraid, the Tower Hills between the Grey Havens and (in later years) the Shire. These towers were built by Gil-galad of Lindon for Elendil in the earliest days of the North-kingdom. Within Elostirion, Elendil placed the Seeing-stone of Emyn Beraid, a palantír that allowed him to see back across the Great Sea into the West, now lost to him and his people. After his time, the towers and their Stone were guarded and maintained by Círdan and the people of Lindon, and became a place of pilgrimage for the Elves (and thus Elves out of Rivendell were sometimes encountered by the Shire-hobbits, travelling west to look into the Elendil Stone in Elostirion).

At the end of the Third Age, the Stone was removed from Elostirion and taken back into the West aboard the White Ship, but the towers remained. This area became part of the Westmarch of the Shire, and the Hobbits settled on Emyn Beraid beneath Elostirion and the other towers, from which they named their new settlement 'Undertowers'.