A millimetre wobble to the left? The sensors will catch that, says MTR's construction manager David Salisbury, though nothing out of the ordinary has been detected so far.

'Can't say there's been none, because these things are so sensitive they'll record every millimetre,' Salisbury said. 'Even temperature changes within the air on a hot, hot day and a cold day will cause these buildings to move slightly.' Rather, he said, the sensors looked for 'continuous trends associated with our excavations'.

If something does happen, he and others will get an instant e-mail alert from a 24-hour automated monitoring system.

It's a serious job - making sure the worst-case scenario of collapsed buildings and a big hole in the ground doesn't come to pass. As if that's not complicated enough, the entire three-kilometre route lies under one of the city's oldest and most densely populated areas.

The building monitoring and surveys were not new, but the techniques used to make sure life on the ground would go on with little disruption made this project uniquely complex and one that would be studied closely in future, Salisbury said.

'In Western District, space comes at a premium,' he said.

While a TBM usually works in a site the size of several football fields, this site at Sai Wo Lane, off Queen's Road West, might fit into only half of one. 'It's a constrained site, completely surrounded by residential areas. It's right up against someone's bedroom,' Salisbury said.

Starting in the summer, they've blasted a hole underground and made space for the 70-metre-long TBM. There was a little shaking with the blasts, and some noise, but now that the machine is working underground, it's business as usual with seafood drying in the wind just steps away from the main site. 'You wouldn't know it's there,' Salisbury said, noting there had been no complaints since the tunnelling started.

As of Wednesday, the TBM was about 100 metres into its one-kilometre journey, leaving behind it a trail of support systems and pipes snaking through the concrete labyrinth. It spits out a steady stream of waste rocks and soil to the surface that will be removed to a treatment plant, while filling gaps in its trail with grout and pressurised slurry. The noise is a low hum, and the cranes clank like any other construction site.

'None of this would have been possible 20 years ago. It would have been much more disruptive with traffic jams, more open construction sites, more noise,' Salisbury said.

Even with the drill and blast, typically noisy and forceful, they're using electronic detonators to create explosions accurate to the millisecond and covering the area with water to minimise noise, vibrations and dust.

Last year, when the MTR began blasting works along the new route, the South China Morning Post reported the disturbance was so slight that even tai chi classes taking place in a nearby park were unaffected.

As the new tunnel will take a different route under Sheung Wan station from old tunnels built years before, the crew will also make use of a tunnel-dismantling machine, the first of its kind in the world.

Of course, all of this is taking place 30 metres underground, so for the most part none of us will feel a thing.

'Nobody ever sees what we do, until we get it wrong,' Salisbury said. 'I have friends who build bridges; they've got great icons to look at. Those of us who work on projects underground don't get that. We just have thousands of people go through in trains who will never see it. We don't mind. We try not to have a chip on our shoulder because of it.'

This part of the track is to be built by early 2013, while the entire West Island Line is to be finished by 2014. The MTR says it is still operating within the HK$15.4 billion budget set in 2009, of which the government provides HK$12.3 billion.