PFC Energy, 25 to 30 trillion cubic feet of recoverable natural gas resources have been discovered in Tanzania since 2010, bringing the total reserves to over 43 trillion cubic feet by the end of 2013. The value of natural gas actually produced in 2013 was US$52.2 million, a 42.7 per cent increase over 2012. Commercial production of gas from the Songo Songo Island field in the Indian Ocean commenced in 2004, thirty years after it was discovered there. Over 35 billion cubic feet of gas was produced from this field in 2013, with proven, probable, and possible reserves totalling 1.1 trillion cubic feet. The gas is transported by pipeline to Dar es Salaam. As of 27 August 2014, TANESCO owed the operator of this field, Orca Exploration Group Inc. A newer natural gas field in Mnazi Bay in 2013 produced about one-seventh of the amount produced near Songo Songo Island but has proven, probable, and possible reserves of 2.2 trillion cubic feet. Virtually all of that gas is being used for electricity generation in Mtwara. The Ruvuma and Nyuna regions of Tanzania have been explored mostly by the discovery company that holds a 75 per cent interest, Aminex, and has shown to hold in excess of 3.5 trillion cubic feet of natural gas. A pipeline connecting offshore natural gas fields to Tanzania's commercial capital Dar es Salaam was completed at the end of April 2015. Tourism Travel and tourism contributed 17.5 per cent of Tanzania's gross domestic product in 2016 and employed 11.0 per cent of the country's labour force (1,189,300 jobs) in 2013. Overall receipts rose from US$1.74 billion in 2004 to US$4.48 billion in 2013, and receipts from international tourists rose from US$1.255 billion in 2010 to US$2 billion in 2016. In 2016, 1,284,279 tourists arrived at Tanzania's borders compared to 590,000 in 2005. The vast majority of tourists visit Zanzibar or a "northern circuit" of Serengeti National Park, the Ngorongoro Conservation Area, Tarangire National Park, Lake Manyara National Park, and Mount Kilimanjaro. In 2013, the most visited national park was Serengeti (452,485 tourists), followed by Manyara (187,773) and Tarangire (165,949). Banking The Bank of Tanzania is the central bank of Tanzania and is primarily responsible for maintaining price stability, with a subsidiary responsibility for issuing the banknotes and coins of the Tanzanian shilling. At the end of 2013, the total assets of the Tanzanian banking industry were