Gruta do Natal (Christmas Cave), also known as Galeria Negra or Gruta do Cavalo, is a cave system in municipality of Praia da Vitória, on the island of Terceira in the Portuguese archipelago of the Azores.

This article relies largely or entirely on a single source. (July 2019)

Quick Facts Location, Coordinates ...

Gruta do Natal
A passage within the Gruta do Natal system
Location	Terceira Island (Azores), Portugal
Coordinates	38°47′28″N 27°17′57″W
Length	697 metres (2,287 ft)
Discovery	1838
Geology	Lava tube, Basalt
Entrances	1
Access	Tours available in season

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The caves formation is unclear.^[1] Its genesis can be explained by the ancient lava fields that developed from fissural vents, that flowed through the valley between the larger volcanic peaks.^[1]

Initially known as the Galeria Negra (Black Gallery), was baptised as the Gruta do Cavalo following the first exploratory investigation, when the explorers discovered the remains of a horse, which may have fallen at the entrance of the cave.^[1] Later, after 25 December 1969, with the assistance of the eastern patriarch, D. José Vieira Alvernaz, islanders began referring the cave as the Gruta do Natal.^[1] It was on this date that they opened the cave to the public, with a simple access-way and rudimentary lighting. Traditionally, a Christmas mass is held at the site, but there have also been baptisms and at least one marriage on the site.^[1]

The sites was the destination of various acts of vandalism, and today its access has been conditional.^[1]

The mountaineering group Os Montanheiros constructed a rustic house (typical of the hay-loft/barns) to support activities during the site's exploration.^[1] The simple structure was also the centre of vandal attacks and criminal activities over its history, resulting in its degradation.^[1] In 1989, the mountaineering group began a project to promote tourist activities at Gruta do Natal, with the construction of a new building, which was integrated into the surrounding landscape.^[1] This building was sued to shelter an electric generator, to provide permanent lighting within the cave; protect the site from vandalism; and provide an interpretative centre for visitors to the site. The building includes washrooms, a generator room, and an access-way and staircase to the cave.^[1] Inaugurated on 1 December 1998, the anniversary date of the mountaineering association, it was opened to the public on 1 June 1999.^[1]

The interior circuit was developed so that visitors would not follow the same trajectory coming and going along the route, while providing information about the histo-cultural traditions of this cave, in addition to the scientific and geological history.^[1] It is a pedagogic meeting-point used by professors and teachers to present the history of the formative forces that occur in the archipelago of the Azores.^[1] The site is open to the public from June to September, every day, throughout the afternoons, with special openings coordinated through the Montanheiros organization by appointment.^[1]

The Gruta do Natal is located near the Picos Gordos, within the Reserva Florestal Natural da Serra de Santa Bárbara e Mistérios Negros (Santa Bárbara and Mistérios Negros Natural Forest Reserve), an humanized area encircled by pasture-lands and forest of Azorean juniper.^[1] With a privileged position, the site is located near a roadway and popular lake-front picnic area of Lagoa do Negro.^[1] The roadway is also an access-way for tourist routes; it is being one of the few caves in the Azores that is visibly identifiable, visible from major thoroughfares and publicly signalled.^[1] Of all the cave systems in the Azores, Gruta do Natal provides the best conditions for tourist exploration, along with the Algar do Carvão system.^[1]

It is a lava tube of 697 metres (2,287 ft) length, with easy access, high ceiling and floor with slight slope.^[1] The interior of the cave includes geological structures such as different examples of lava, estalcites and lateral protuberances.^[1]