This structure has been explored on many occasions in the past; its elaborate decoration is well known. Although it was damaged by the bunker (the covering slab and four chamber pillars on the southern side were destroyed), it has been well restored, in particular thanks to the casts kept in the Carnac museum.

The structure is "semi-megalithic" as large drystone walling panels are inserted between the slabs, forming the walls of the crypt.

The seven-metre-long passage leads to an almost square chamber, just over three metres long. The passage covering had long since disappeared, but before the war, the chamber was still protected by a large flaky granite rock slab of local origin, that was destroyed when the bunker was built.

A colossal portal frames the entrance on the monument's main face. It consists of three imported orthogneiss blocks. The two 2.4m high pillars have been levelled by pecking and the lintel (found broken and restored) is a splendid 4.5-metre-long spindle-shaped stone, bearing a number of cupmarks.

Because of its exceptional and particularly colossal shape, the portal has sometimes been considered as a Roman addition. However, in spite of many signs that the monument was in fact extensively re-used in Roman times, excavations have not provided any stratigrafic evidence to back up the theory.

The artefacts found in the grave are very fragmentary and disparate, indicating various different uses for the monument.

Dolmen IIIa entrance and its monumental orthogneiss portal. On both sides, two large worked stones, horn-shaped for the one and crosier shaped for the other.