Submarine Mines

Most of Nova Scotia’s coal has been mined from under the ocean’s floor, in some cases, several miles from shore.

While the thought of being so far underwater might send chills down your spine, submarine mines (mines under the ocean’s floor) were arguably no more or less dangerous than underground mines in the historical era. In fact, sea water was rarely a significant issue in them.

Most of the mining in the Sydney Coalfield was submarine because 98% of the coalfield is underwater – it extends almost to the south coast of Newfoundland. The coalfield, which extends from Cape Morien to Cape Dauphin, and 300 kilometres offshore, has hosted about 100 mines and produced more coal than all other Nova Scotia coalfields combined.


Water enters most submarine and underground mines through the surrounding rock, but it is usually in amounts small enough that it can be managed with pumping systems.

The fact that most mines constantly “make” water is illustrated by this anecdote from a 1925 MacLean’s Magazine article called “Toilers under the Sea.” In it a Cape Breton coal miner recounts a joke played on a miner named Sam: “He’d never worked in an undersea mine, and it made him a little nervous. The face he was put to work at was kind of wet, and every morning, when he came on the job, there was a pool o’ water at the foot o’ the seam. Some o’ the byes used to spill salt in it on the sly then get him to taste it, makin’ believe the sea was comin’ in. Well, he got used to that, but one day they tried a new stunt. His buttie brought down a live herring in his dinner pail and dropped it in the pool, then called Sam over to look at it.”


As with mining underground, mining under the ocean’s floor requires careful engineering and planning. The “room and pillar” method was often used, a mining system in which "rooms" of ore are dug out while "pillars" of untouched material are left to support the mine’s roof and the weight of the rock and ocean floor above.

Louis Frost, a mining engineer who worked for the Dominion Coal Company, wrote in the 1960s that it was common practice in the Sydney Coalfield to limit the width of rooms to a maximum of 20 feet and to extract a maximum of 59% of the coal, leaving at least 41% behind in pillars to support the mine’s roof.

Another frequently used method in the Sydney Coalfield was longwall mining, in which a machine sheared coal off the coal face in a single, continuous operation and carried it on a conveyor belt back to the main mine tunnels. As a longwall miner advances, the roof behind the machine is allowed to collapse in a controlled manner. The system is safe for miners because they remain outside the section being mined and let the machine do the work.


It is important for submarine mines to have sufficient rock cover – rock between the mine’s roof and the sea floor - to help support the weight of the ocean floor and to minimize the risk of sea water leaking through faults or fractures into the mine. (For example, lack of rock cover was a problem at the Victoria coal mines:

Since the 1800s, government regulations have specified the minimum amount of rock cover required. A 1974 report compared the regulations in place at that time in Nova Scotia with other jurisdictions that did submarine coal mining. In Nova Scotia, 180 feet of rock cover was required. 120 feet was required in Australia, 197 feet in the UK, 197 feet in Japan and 230 feet in Chile.

The regulation seems to have worked since there were almost no significant flooding issues in submarine mines. However, the problem with regulating a minimum distance is that it does not take into account an area’s unique geology and how that affects the potential for flooding.

This is highlighted by a 1909 flood in the Mabou coal mine. The mine fulfilled the regulations in place at the time but the geology in Western Cape Breton contained more faults than the coalfields on the eastern side of Cape Breton. (In geology, a fault is a fracture, or zone of fractures, between two blocks of rock. Faults are caused by geological forces like tectonic plate movement and they allow the blocks of rock to move relative to each other.)

In the case of the Mabou flood, cracks in the mine’s rock cover allowed sea water to enter the mine. The regulation for rock cover was insufficient due to the area’s geology. Fortunately, the flood was relatively minor and no one was hurt. Mining was continued elsewhere in the mine (


In the case of mines operating under land, additional shafts or decline tunnels can be sunk when extraction reaches a significant distance from the shaft bottom, but that obviously is not an option when the mine is under the ocean floor – it can only be accessed from shafts and tunnels on land. As a result, mines eventually shut down as they get further from shore because the time and cost required to move equipment, coal and miners from the mine mouth on shore to the working face becomes too great.

For example, the Dominion No. 4 mine (aka Caledonia) extended 5.8 kilometres from shore when it closed in 1961. Additional extraction would likely have been uneconomical. (See the mine’s story at

The furthest from shore a Nova Scotia submarine coal mine reached was the Prince mine in Point Aconi. Its tunnels extended almost eight kilometres under the ocean and it took miners 45 minutes to be transported to the coal face in its final years (

Mining was often a dangerous job historically. However, Nova Scotia’s mining and quarrying industry has reduced its injury rate by 90% since the Westray inquiry report was released in 1997, making mining one of the safer industries in the province. We believe the most important thing to come out of a mine is the miner, and our modern safety record reflects this.