But if no stream issues visibly from the lake, numerous springs rise at the bottom of the bank that bounds it, due doubtless to filtration through the scoria, and unite to form a current sufficient to turn a mill before it reaches the Loire distant three-quarters of a mile.
This beautiful tarn, 330 feet deep in the middle, has been menaced more than once. The lake belonged in the Middle Ages to the Chartreuse of Bonnefoy, the ruins of which are in the neighbourhood, and which was founded in 1156 by a Seigneur of Mézenc. The Carthusians used the lake not only as a fishpond to furnish their table, but also as a reservoir for the irrigation of their meadows by means of canals.
In 1793 it ran its first risk. With the laudable object of draining marshy land and rendering such lake bottoms as could be reclaimed serviceable for culture, a law was passed on the 14th to 16th Frimaire (4th to 6th December), and at the beginning of 1794 the Citizen Auzillon was deputed to inspect and report on Issarlès. But he was driven back by storms of snow, and obliged to postpone his examination of the lake. He started again in July, and was accompanied by the deputy sent down from Paris to organise an expedition for hunting out and bringing to the lamp-post or the guillotine the priests and royalists who were supposed to be in concealment in the neighbourhood. Auzillon declared in his report that the draining of the lake would cause an unwarrantable expense and prove unprofitable. It lay, he said, in the crater of an extinct volcano, and that he had been unable by sounding to discover the depth.
The lake has been again threatened, this time with conversion into a reservoir for the water-supply of factories, to be established at a lower level.
