Every cutting tool on a boring machine wears out.
That is not a fault, it is the job. Rock grinds steel away, and after a few thousand feet the discs are flat and the machine slows to nothing.
On land the answer is easy. You stop, open the head, and somebody walks in and changes them.
Under the sea that stops being simple.
The face in front of the cutters is not held up by rock. It is held up by pressure.
And you cannot turn that off.
The chamber has to be at the pressure of the seabed
A closed machine keeps a sealed chamber pressed against the excavation, filled with slurry or compressed air at whatever the ground and water outside are pushing.
Reduce that and the face collapses into the machine. So anybody going forward to reach the cutters has to enter at the same pressure the ground is at.
That is diving, whatever the surroundings look like, and at shallow depths it is ordinary diving on compressed air.
The arithmetic breaks at depth. At the pressure this machine works under, a single short visit would need days of decompression afterward, which makes the whole idea pointless.
Saturation is the way around it. Past a certain time under pressure the body stops absorbing gas, and the decompression stops getting longer.
So the crew goes to pressure once and stays there, living in a sealed habitat on the machine and shuttling to the cutterhead for shifts.
They come back to normal pressure once, at the end.
The machines and the water above them
The crossing runs between Ningbo on the mainland and the Zhoushan islands off the Chinese east coast.
Two boring machines are driving it, each with an excavation diameter of about 48 feet and a weight near 4,800 tons.
The tunnel runs 10 miles in total, of which 7 are bored by machine.
The ground is the difficult part. About three miles of it alternates between soft and hard layers, and the hard rock reaches 27,700 pounds per square inch.
That is several times the strength of structural concrete, and it is what eats the cutters.
Above all of it sits one of the busiest stretches of shipping water anywhere, which is the whole reason for going underneath.
Twenty one days at eight and a half bar
The intervention was completed and announced in August of 2026.
It happened 302 feet below sea level, where water and soil press at 123 pounds per square inch, and about 6,500 feet of the drive sits above 87.
Crews inspected and replaced 104 cutting tools over 21 days without ever bringing the face pressure down.
The habitat is described as the first saturation system built in China for shield tunneling, and it has been nicknamed a deep sea space station.
The chief mechanical engineer said crews can work eight hours at a stretch, allowing round the clock shifts for as long as 28 days.
Subsea boring on this drive began in the summer of 2025, and the wider railway is targeted for 2028.
Which record this actually is
The claim needs its qualifier. This is the longest undersea tunnel for high speed rail, which is a narrow category and not the longest undersea tunnel of any kind.
The depth record belongs elsewhere. A machine on a line further south has reached about 371 feet below the seabed under an estuary, and the two keep getting merged into one story.
That tunnel is shorter, uses a different machine and serves a different city pair, so its figures do not apply here.
The 21 day figure also covers one intervention rather than the whole drive. Cutter changes on a bore this long happen repeatedly, and the totals get added up carelessly, in the way that numbers around a major crossing or a very large machine tend to collect over time.
The pressure and tool count come from the contractor’s own account, as the state media report of the operation sets out.
Why not simply put it all on the bridge
There is already a bridge on this route, and it is a remarkable one.
It carries two railway tracks between three highway lanes each side, on a main span of 4,882 feet.
So the question is fair. If a bridge can cross one channel, why bore under the next.
The answer is the shipping. This is a deep water approach used by very large vessels, and a bridge would need towers and clearance that push the cost past the tunnel.
The finished line runs 48 miles with seven stations, at a cost near 4.1 billion dollars, and it cuts the trip from the mainland to the islands to 26 minutes, as the rail sector project file records.
The boring machine was never the hard part of this crossing.
Getting a person in front of it was.
