"The chamber is unique both because of its size and the manner in which it is secured. It was worked for almost a hundred years (from late 17th century to mid-18th century) in a vertical block of green salt. At the turn of the 20th century, an impressive wooden casing was constructed in the excavation. It rests on two sheaf pillars linked by wooden stairs and galleries. At a later date, the chamber was reinforced by a salt-block and a brick-and-concrete wall." (From the official website).