the 18th of February, about three o'clock in the morning
And Conseil , reassured , returned to the study of the bank , which the Nautilus was skirting at a moderate speed . During the night of the 16th and 17th February we had entered the second Mediterranean basin , the greatest depth of which was 1,450 fathoms . The Nautilus , by the action of its crew , slid down the inclined planes and buried itself in the lowest depths of the sea . On the 18th of February , about three o'clock in the morning , we were at the entrance of the Straits of Gibraltar . There once existed two currents : an upper one , long since recognised , which conveys the waters of the ocean into the basin of the Mediterranean ; and a lower counter-current , which reasoning has now shown to exist . Indeed , the volume of water in the Mediterranean , incessantly added to by the waves of the Atlantic and by rivers falling into it , would each year raise the level of this sea , for its evaporation is not sufficient to restore the equilibrium . As it is not so , we must necessarily admit the existence of an under-current , which empties into the basin of the Atlantic through the Straits of Gibraltar the surplus waters of the Mediterranean .