Meneltarma

Near to the central part of Númenor within the province called Mittalmar stood a tall mountain called the Meneltarma, Pillar of the Heavens, sacred to the worship of Eru Ilúvatar.

Details and Comments

....a winding spiral road was made upon it, beginning at its foot upon the south, and ending below the lip of the summit upon the north. For the summit was somewhat flattened and depressed, and could contain a great multitude but it remained untouched by hands throughout the history of Númenor. No building, no raised altar, not even a pile of un-dressed stones, ever stood there and no other likeness of a temple did the Númenóreans possess in all the days of their grace, until the coming of Sauron. Unfinished Tales/A Description of the Island of Númenor

This was a sacred place for the Númenóreans and for centuries they preserved some specific traditions:

There no tool or weapon had ever been borne and there none might speak any word, save the King only. Thrice only in each year the King spoke, offering prayer for the coming year at the Erukyermë in the first days of spring, praise of Eru Ilúvatar at the Erulaitalë in midsummer, and thanksgiving to him at the Eruhantalë at the end of autumn. At these times the King ascended the mountain on foot followed by a great concourse of the people, clad in white and garlanded, but silent. At other times the people were free to climb to the summit alone or in company, but it is said that the silence was so great that even a stranger ignorant of Númenor and all its history, if he were transported thither, would not have dared to speak aloud. No bird ever came there, save only eagles. If anyone approached the summit, at once three eagles would appear and alight upon three rocks near to the western edge; but at the times of the Three Prayers they did not descend, remaining in the sky and hovering above the people They were called the Witnesses of Manwë, and they were believed to be sent by him from Aman to keep watch upon the Holy Mountain and upon all the land. Unfinished Tales/A Description of the Island of Númenor

The sacred mountain was also guarding the eternal rest of the dead kings:

The base of the Meneltarma sloped gently into the surrounding plain, but it extended, after the fashion of roots, five 1ong low ridges outwards in the direction of the five promontories of the land; and these were called Tarmasundar, the Roots of the Pillar. Along the crest of the south-western ridge the climbing road approached the mountain; and between this ridge and that on the south-east the land went down into a shallow valley. That was named Noirinan, the Valley of the Tombs; for at its head chambers were cut in the rock at the base of the mountain, in which were the tombs of the Kings and Queens of Númenor. Unfinished Tales/A Description of the Island of Númenor