In 1983 they started excavating in the bottom of the gravel pit.
The volunteers assist in releasing the hatchings and excavating the nests to determine the hatching success rate.
The subdrift system requires a smaller amount of narrow work in excavating the necessary haulage roads, and is therefore better adapted to hard ores in which such narrow work is expensive.
The hilly aspect of several parts of the central plateau is not due to foldings of the strata, which for the most part appear to be horizontal, but chiefly to the excavating action of the rivers, whose valleys are deeply eroded in the plateau, especially on its borders.
These lowlands, dotted over with numberless marshes and lakes, seem to have emerged from the sea at a quite recent geological period; the rivers that meander across them are still excavating their valleys.