Anta de Adrenunes

The Anta de Adrenunes, located on top of a hill at 426 metres above sea level, in the municipality of Sintra, within Sintra-Cascais Natural Park, Lisbon District, Portugal, is believed to be a Stone Age burial chamber or megalithic monument. It is a structure consisting of a cluster of granite stones, between which there is a gallery about 5 metres high that is surmounted by monoliths that rest horizontally on vertical stones. The passage is thought to have served as a collective necropolis or dolmen during the megalithic period although no artifacts or burial chambers have been found to prove this.

Source: Wikipedia