Downhole operation, formerly known as "underground operation", refers to the passage of people through the channel cut from the ground to enter the underground at a certain depth from the surface to engage in specific purpose production activities. Which of the following does not belong to underground operations? ['Construction of subway engineering.', 'Maintenance of downhole equipment and equipment, water, electricity, wind transmission and pipeline maintenance work, etc.', 'Bridge between two mountains.', 'Coal mine, tungsten mine and other non-opencast mining.']