The Cairn of Barnenez is the largest Megalithic mausoleum in Europe. It dates from the early Neolithic Age is considered one of the earliest megalithic monuments in Europe. It is also remarkable for the presence of megalithic art. Radiocarbon dates indicate that the first phase of the monument was erected between 4850 and 4250 BC, and the second phase between 4450 and 4000 BC. Pottery found in and around the monument indicates that it underwent a period of reuse in the Bronze Age, in the 3rd millennium BC.
The cairn was first mapped in 1807, in the context of the Napoleonic cadaster. Its first scientific recognition took place in the context of an academic congress in Morlaix in 1850, when it was classified as a tumulus. Privately owned until the 1950s, the cairn was used as a quarry for paving stones. This activity, which threatened to destroy the monument, was only halted after the discovery of several of its chambers in the 1950s. The local community then took control of the site. The cairn was restored between 1954 and 1968. At the same time, vegetation was removed from the mound and systematic excavation took place in and around the monument.
Today, the Barnenez cairn is 72 m long, up to 25 m wide and over 8 m high. It is built of 13,000 to 14,000 tons of stone. It contains 11 chambers entered by separate passages. The mound has steep facades and a stepped profile. Several internal walls either represent earlier facades or served the stability of the structure. The cairn consists of relatively small blocks of stone, with only the chambers being truly megalithic in character. The monument overlooks the Bay of Morlaix, probably a fertile coastal plain at the time of its erection.
Engraved symbols occur in several of the chambers and passages. They depict bows, axes, wave symbols or snakes and a repeated U-shaped sign. One of the carved slabs is in secondary use was originally part of a different structure, an interesting parallel to the situation in several other such monuments, including Gavrinis. The symbols on the engraved blocks resemble those found in other megalithic monuments in Brittany; in broader terms they belong to the cultural phenomenon described as megalithic art. One of the recurring symbols is sometimes interpreted as an anthropomorphic depiction (the so-called \'Dolmen Goddess\').
An exhibition in the modern entrance building explains the results of scientific excavation and displays some objects from the site.