A South Korean company is operating the country’s largest indoor “smart farm” inside a former road tunnel. The farm produces fruits and vegetables hydroponically, which means the plants do not grow in the ground. Instead, they grow on vertical equipment that provides a continuous flow of water. Indoor farms use artificial light instead of sunlight. There are other vertical smart farms in South Korea. But this one built by the company NextOn about 200 kilometers south of Seoul - is the nation’s biggest. It is also one of the largest indoor vertical farms in the world, with a total area of 2,300 square meters. But what makes the farm even more unusual is the fact that it sits inside a huge tunnel. The tunnel was built in 1970 for one of South Korea’s first major highways, but it closed down in 2002.