The drill that turns beneath an unopened road

A drill bit can travel 500 feet under a busy highway, turn a sharp corner, and come out the exact side of a manhole cover without the cars above ever feeling a bump. It doesn't just go in a straight line like a hand drill. The machine actually steers itself through solid earth using a slanted face that acts like a rudder.

If the operator spins the drill rod constantly, the bit goes straight because the slant hits every side of the hole evenly. But when they need to turn, they stop the spinning and just push. The angled head slides against the dirt and veers off in one direction. By watching a radio beacon tucked inside the drill head, the crew above ground can guide the bit around buried power lines or bridge foundations like they're playing a high-stakes game of Operation.

The hole stays open because of a soup called drilling mud. This isn't just wet dirt—it's a high-tech mix of water and bentonite clay that acts like liquid scaffolding. It's slippery enough to lubricate the pipe, but thick enough to plaster the walls of the tunnel so the whole thing doesn't cave in on itself.

This mud also carries all the ground-up rocks and soil back to the surface. But if the pressure gets too high or the ground is too full of cracks, the mud can take a detour. It's called an "inadvertent return," which is a polite way of saying the mud just exploded through someone's lawn or the bottom of a creek bed. When that happens, everything stops until the mess is contained.

Once the tiny pilot hole is finished, the drill bit pops out of the ground at the far end. The crew swaps the bit for a giant, spiked cone called a reamer and hitches the new permanent pipe to the back of it. As the machine pulls the rod back toward the start, the reamer chews the hole wider and drags the new pipe right into the space it just cleared.

It's a strange way to build a city, essentially knitting a web of utilities through the dirt without ever touching the surface. But the ground isn't always empty space. Sometimes the drill hits a pocket of loose gravel that acts like marbles, making the steering head spin uselessly while the mud vanishes into the void.