A tour down to the second tunnel of the Ho Chi Minh City metro system, the longest underground passage in Vietnam, provides awe of its technical aspect and working environment.

This underground route, stretching from Ba Son Terminal to the Municipal Theater Terminal in District 1, Ho Chi Minh City, began to be constructed on January 26 after the first tunnel came into existence in October last year, and is planned to be completed in June.

A descent down several steep flights of stairs took a visitor to the building scene more than ten meters below the ground.

The tunnel is spacious, closely fitted with prefabricated slabs of concrete and muffled sounds of purring machines from the nearby parallel subterranean passage in the making.

Engineers were taking copious notes of the site operation while concrete blocks were being carried from the entrance to the spot of a Japanese-produced tunnel boring machine (TBM).

The robot's drilling bits are the adult’s big-toe size, about 15 centimeters long, and mounted around a circular steel frame.

After digging 1.2 meters, the robot stops to install concrete slabs, weighing roughly 3,000 kilograms each and 300 millimeters thick, onto the tunnel walls and floor.

Despite the slabs' bulk and lack of room, this specialized machine managed to position them with exactitude and ease as if it were handling jigsaw pieces, leaving workers with the job of driving bolts for reinforcement.

The machine is programmed to ensure that the tunnel is water-proof and able to withstand high pressure or strong shocks.

Engineers said that six slabs of concrete have been fixed in this way every day until now.

The passage's interior was not dim since it was equipped with electric lights, or even high-powered ones, all the way through, while the robot had red and yellow lights.

Contrary to usual imagination of a construction site, this place had no dust, odor, and rock or earth spread widely, and the workers were not drenched with sweat transferring the materials.

The TBM robot can simultaneously drill and spray water and additives to the rock, which is then conveyed to the ground in a closed system, the engineers said.

Temperature inside the tunnel is maintained equal to that of the outside thanks to sizable ventilation systems.

To qualify as staff here, who numbered 30, the workers had to spend averagely three months acquiring knowledge and skills necessary for the underground environment, while advanced engineers must also know Japanese to work with experts from this country.

The site motto, 'Think safety, think family,' is printed on signs located sparsely along the tunnel, effectively indicative of the importance of safety here.

The workers mostly used nonverbal gestures and are supposed to eschew all personal business.

A piece of concrete is moved inside the tunnel. Photo: Tuoi Tre

The central operation positioned inside the tunnel. Photo: Tuoi Tre

The underground passage is illuminated with lights. Photo: Tuoi Tre

Pipes channeling rock and earth to the ground. Photo: Tuoi Tre

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