Strabo Geography 16.2.42

[42] The Lake Sirbonis ¹ is of great extent. Some say that it is 1000 stadia in circumference. It stretches along the coast, to the distance of a little more than 200 stadia. It is deep, and the water is exceedingly heavy, so that no person can dive into it; if any one wades into it up to the waist, and attempts to move forward, he is immediately lifted out of the water ² It abounds with asphaltus, which rises, not however at any regular seasons, in bubbles, like boiling water, from the middle of the deepest part. The surface is convex, and presents the appearance of a hillock. Together with the asphaltus, there ascends a great quantity of sooty vapour, not perceptible to the eye, which tarnishes copper, silver, and everything bright—even gold. The neighbouring people know by the tarnishing of their vessels that the asphaltus is beginning to rise, and they prepare to collect it by means of rafts composed of reeds. The asphaltus is a clod of earth, liquefied by heat; the air forces it to the surface, where it spreads itself. It is again changed into so firm and solid a mass by cold water, such as the water of the lake, that it requires cutting or chopping (for use). It floats upon the water, which, as I have described, does not admit of diving or immersion, but lifts up the person who goes into it. Those who go on rafts for the asphaltus cut it in pieces, and take away as much as they are able to carry.