 Sarnia 's grain elevator , which is the sixth largest currently operating in Canada , was built after the dredging of Sarnia Harbour in 1927 . Two short years later , grain shipments had become an important part of Sarnia 's economy . The grain elevator rises above the harbour , and next to it is the slip for the numerous bulk carriers and other ships that are part of the shipping industry that includes vessels from all over the world . The waterway between Detroit and Sarnia is one of the world 's busiest , as indicated by the average of 78 @,@ <unk> @,@ 900 tonnes ( 87 @,@ 020 @,@ 800 short tons ; 77 @,@ 697 @,@ 100 long tons ) of shipping that annually travelled the river going in both directions during the period 1993 – 2002 . Lake <unk> and <unk> ships , which are known as " <unk> , " pass up and down the river at the rate of about one every seven minutes during the shipping season . During this same period , The Paul M. <unk> Tunnel , which was named after the retired president of <unk> in 2004 , was bored and began operation in 1995 . It accommodates double @-@ stacked rail cars and is located next to the original tunnel , which has been sealed . 
