further to the west , and north from the great continent of tharsis , we beheld the immense oval-shaped land of thaumasia containing in its centre the celebrated lake of the sun , a circular body of water not less than 500 miles in diameter , with dozens of great canals running away from it like the spokes of a wheel in every direction , thus connecting it with the ocean which surrounds it on the south and east , and with the still larger canals that encircle it toward the north and west .