A giant indoor farm in China is breeding 6 billion cockroaches a year. Here's why

Long, narrowly spaced rows of shelves fill a multi-storey building about the size of two sports fields. The shelves are lined with open containers of food and water. 

It is warm, humid and dark all year round, with freedom to roam to find food and reproduce. Fully sealed like a prison, it has strict limitations on access to visitors. From birth to death, inhabitants never see the sun.