Noirinan

The Meneltarma, the Pillar of Heaven, was a great mountain that rose alone out of the plains that formed the central parts of Númenor, held to be a holy place by the people of that land. At its base, five long ridges ran out into the surrounding lands, and between these five Roots of the Pillar were shallow valleys. In the valley that opened to the south were the springs of the river Siril, the greatest of the rivers of Númenor, which flowed out the valley and on into the south.

It was this valley that was chosen by the Númenóreans as the resting place of their Kings and Queens. In the rock at the head of the valley, they carved tombs into the base of the mountain itself, and thus the southward valley came to be called Noirinan, the Valley of the Tombs. Noirinan was an important part of Númenórean culture: the first main road constructed by the Men of Númenor ran from the royal city of Armenelos to the Valley of the Tombs, before running up the ridge to the west to reach the summit of the Meneltarma.

We have little detail about the tombs themselves, apart from a brief reference in Akallabêth, where we're told that the entombed Kings and Queens '...slept now in their deep tombs under the mount of Meneltarma, lying upon beds of gold.' If the tombs were as deep under the mountain as this suggests, that implies that they were not buried directly in the valley itself, but rather that the excavations at the head of Noirinan formed entrances to underground tombs deeper under the rock of the Meneltarma.