Strabo Geography 3.5.7

[7] Polybius relates that there is a spring within the temple of Hercules at Gades, having a descent of a few steps to fresh water, which is affected in a manner the reverse of the sea-tides, subsiding at the flow of the tide, and springing at the ebb. He assigns as the cause of this phenomenon, that air rises from the interior to the surface of the earth; when this surface is covered by the waves, at the rising of the sea, the air is deprived of its ordinary vents, and returns to the interior, stopping up the passages of the spring, and causing a want of water, but when the surface is again laid bare, the air having a direct exit liberates the channels which feed the spring, so that it gushes freely. Artemidorus rejects this explanation, and substitutes one of his own, recording at the same time the opinion of the historian Silanus; but neither one or other of their views seems to me worth relating, since both he and Silanus were ignorant in regard to these matters. Posidonius asserts that the entire account is false, and adds that there are two wells in the temple of Hercules, and a third in the city. That the smaller of the two in the temple of Hercules, if drawn from frequently, will become for a time exhausted, but that on ceasing to draw from it, it fills again: while in regard to the larger, it may be drawn from during the whole day; that it is true it becomes lower, like all other wells, but that it fills again during the night when drawing ceases. [He adds] that the ebb tide frequently happening to occur during the period of its re-filling, gave rise to the groundless belief of the inhabitants as to its being affected in an opposite manner [to the tides of the ocean]. However it is not only related by him that it is a commonly believed fact, but we have received it from tradition as much referred to amongst paradoxes. ¹ We have likewise heard that there are wells both within the city and also in the gardens without, but that on account of the inferiority of this water, tanks are generally constructed throughout the city for the supply of water: whether likewise any of these reservoirs give any signs of being affected in an opposite manner to the tides, we know not. If such be the case, the causes thereof should be received as amongst phenomena hard to be explained. It is likely that Polybius may have assigned the proper reason; but it is also likely that certain of the channels of the springs being damped outside become relaxed, and so let the water run out into the surrounding land, instead of forcing it along its ancient passage to the spring; and there will of course be moisture when the tide overflows. ² But if, as Athenodorus asserts, the ebb and flow resemble the inspiration and expiration of the breath, it is possible that some of the currents of water which naturally have an efflux on to the surface of the earth, through various channels, the mouths of which we denominate springs and fountains, are by other channels drawn towards the depths of the sea, and raise it, so as to produce a flood-tide; when the expiration is sufficient, they leave off the course in which they are then flowing, and again revert to their former direction, when that again takes a change. ³