In an upland valley in Kumamoto Prefecture, a huge, dark-silver-colored flat building is home to silkworms that are raised all year round.

Believed to be one of the world's biggest bioclean silkworm factories, it was completed in April 2017 at a cost of ¥2.3 billion ($21 million), according to Kumamoto-based Atsumaru Holdings Co., one of its operators.

The Silk on Valley project in Yamaga, located on the site of a former elementary school in a basin surrounded by mountains, is part of efforts to revitalize Japan's once-thriving sericulture industry.