Especially in the early days, sandhogs were primarily associated with compressed-air work, and might move through caissons in air locks like this one (the metal cylinder with rivets) to safely pass between areas of differing air pressure. When you start digging, or "sinking," a shaft, you usually have to first pass through some earth before reaching bedrock. Earth (soil, sand, shale) has ground water running through it, making it likely to collapse. Caissons are enclosed structures in the shape of the shaft, open on the bottom and filled with compressed air, that create working chambers. As sandhogs removed the earth inside the chamber and the ground level inside got lower, the caisson would also sink down. This is a shaft for City Water Tunnel No. 1, the 18-mile-long conduit built at the same time as the Catskill Aqueduct to distribute clean drinking water toward all five boroughs.

Construction on the tunnel lasted from 1911 to 1917, and employed thousands of sandhogs who tackled other underground jobs, like using a tripod drill to make holes in the bench, or floor, of the tunnel for explosives to be inserted. The drill-and-blast method had some variations but always followed the sequence of drilling holes, filling them with explosives, blasting the rock out, clearing out the broken-up rubble (AKA muck), and starting over again. Excavation was an around-the-clock operation performed in eight-hour shifts.

After a dynamite blast, these sandhogs removed rubble with the help of a mucking machine that sent muck on a conveyor belt to locomotive cars that then transported it through the tunnel to a shaft. Muckers were paid $2 per 8-hour shift.

At the bottom of the shaft, on the cage landing platform 700 feet below Manhattan, sandhogs prepared loaded muck cars to be hoisted out of the tunnel for disposal. They used the air signal on the left to transmit a code of whistles to hoist operators above indicating whether people or materials were on their way up. From the top of the shaft, muck was hauled by horse-drawn wagons to scows in the East River and probably dropped into the ocean.

Sometimes mules carried muck cars through the tunnel!

The Board of Water Supply made some cartoon-like drawings to show methods for tunneling operations. This one illustrates tunnel excavation by drill-and-blast method, with muckers on a platform using wheelbarrows to dump rock into rail cars.

Sometimes drills were installed on platforms, or "jumbos," in this case raising the operators and tools to cut a slot into the rock at the foot of a tunnel shaft. These Radialax drills rotated in an arc. You can imagine the noise, darkness, and potential dangers in this airless space.

Even though safety standards were minimal in the early 20th century, cages that carried workers hundreds of feet up and down the shaft did have safety gates.

After tunnels were excavated, concreting could begin. Storage battery locomotives hauled cars loaded with concrete for the tunnel lining.

These sandhogs posed—and some cracked a smile!—while placing concrete.

After concreting, workers operated a device for forcing grout behind and around the concrete-lined tunnel, filling voids between the rock and the tunnel lining.

Another drawing illustrated grouting operations. City Water Tunnel No. 1 was completed in 1917, and work on the 2nd tunnel began ten years later.

City Water Tunnel No. 2 would be 20 miles long, running directly from the Bronx to Queens under the East River and then into Brooklyn.

Excavation methods were similar, but safety measures had improved. Some, though not all, workers wore a sort of early hard hat with a floppy leather brim.

Geological conditions with soft ground and water-bearing rock at this point of the tunnel were so trying and difficult that seven months were required to advance the tunnel only 100 feet.

Workers grouted the concrete tunnel lining--its smooth surface allows water to flow quickly. Construction of the entire tunnel, from Yonkers to south Brooklyn, lasted from 1929 through 1935. It then became the primary means of water delivery for Queens, Brooklyn, and Staten Island.

Work on City Water Tunnel No. 3 began in 1970 with the goal of improving city-wide distribution and creating redundancy so the older tunnels could be taken out of service for inspection and repair. It was the largest capital construction project ever undertaken by the City and the biggest job for the sandhogs. The first stage of the tunnel was completed in 1998, entirely excavated using the drill-and-blast method, as shown here by sandhogs operating a crawler-type drill jumbo with hydraulic drills to make holes for explosives. The drilling noise was deafening. Later, a shout of “Fire in the hole!” signaled that the explosives were about to be detonated, blasting away tons of rock. During the first two stages of the tunnel about 82 million cubic feet of rock was excavated, enough to fill both the Empire State Building and Yankee Stadium!

Scaling was another essential and dangerous sandhog task. After blasting and before mucking, a crew of scalers scraped loose debris from the tunnel ceiling to protect workers from falling rock, removing it intentionally to prevent an accident.

In the 1990s, DEP began using tunnel boring machines (AKA TBMs or moles) instead of drill-and-blast to dig parts of City Water Tunnel No. 3, and sandhogs adapted to the new technology. The large cylindrical machine has a face holding cutters, round blades that weigh several hundred pounds each. The cutters rotate to crush the rock face of the tunnel. The TBM creates a smoother tunnel with limited disturbance to the rock and is safer because it avoids the use of explosives. It's also more efficient and can cut a narrower, more precise, tunnel, so less concrete has to be used for the lining.

23 workers--sandhogs, operating engineers, and DEP and contractor's employees--lost their lives during construction of City Water Tunnel No. 3. DEP and the Local 147 Sandhogs Union worked together on this memorial which was completed in 2007. Each of the 23 manhole covers embedded in the plaza bears the name of a person and their year of death. Fittingly, there is also a drinking fountain. There have been no worksite fatalities on the project since 1997.

Another recent tunneling project was the Staten Island Siphon. After tunnel excavation to Brooklyn was completed in March 2015, sandhogs posed in front of the TBM cutter head. The new siphon was excavated at a depth of 100 feet and replaced the old Narrows Siphons 1 and 2. It serves as the backup water feed for Staten Island. The City's water supply and distribution system are continuously improved and expanded, and if the work is underground, sandhogs will be there.