Water, it&#x27;s fine if it takes 10 years to move water from point A to point B as long as you don&#x27;t need to build and maintain a pipe.  Also, they are not solid rock or water, it&#x27;s more like gravel with water in-between but the flow rate can get fairly high due to size even if the flow speed is very low.  1 mile per day is slow but * 365 days * 10 years that&#x27;s a long way.