Only with the travels of Pytheas of Marseilles along the North Atlantic coasts, the expedition of Alexander, and Nearchus’ voyage and exploration of the mouths of the Indus and coasts of Beluchistan and Mekran did the Greeks gain any adequate knowledge of tidal phenomena; for the tides of the Mediterranean, except in a few places, are so low as to be almost negligible.[108] Eratosthenes thought that the currents through narrows in the Mediterranean are caused by variations in the relative levels of the sea at either end of the channels and that these variations are a response of the sea to fluctuations of the tides in the ocean beyond the Pillars of Hercules.[109] As early as the third century before the Christian era the Greeks had understood the relation of the moon’s phases to the ebb and flood, but certainly not much earlier, for Aristotle appears to have been ignorant of it.[110] Posidonius was the first to give a full account of the manner in which the moon and sun regulate the tides.[111] He had accurate knowledge of the diurnal, the monthly, and perhaps the annual tidal periods,[112] a knowledge which formed a bulwark of the structure of his astrology. Pliny also believed that the tides were caused by lunar influence and described the three periods with even greater accuracy than Posidonius.[113] He recognized that the tides must correspond to a lunisolar cycle of one hundred lunations, or eight years, an astronomical cycle that had long been familiar to the Greeks.[114] He included in his account an astute observation that the tides, like everything else on the earth’s surface depending on celestial controls, tend to drag behind the time when these controls are exerted.[115] Seneca does not try to explain the tides; he mentions them only incidentally in connection with a graphic description of the terrible deluge that will overwhelm the earth at the end of the Great Winter. Though in some respects like the spring tides at the equinoxes, when the sun and moon are in conjunction, this flood will be bound by no law of nature and will have no curb to its fury.[116] Macrobius’ explanation of the tides,[117] which was copied by many later writers, though ingenious, was not founded on actual knowledge or observation. He said that the ebb and flood are caused by the impact of the opposing currents of the two ocean belts which encircle the earth, and, with Eratosthenes, he thought that the tide of the Mediterranean is a repercussion of the ocean tides. Indeed, after the time of Pliny there was no addition to the scientific understanding of tidal phenomena until the eighth century. SUBTERRANEAN CHANNELS