Deep inside a forest in drone-infested eastern Ukraine, I stepped onto a small metal platform and descended into a postapocalyptic world of modern warfare, where the only safe place is underground.

When the elevator stopped, a vast subterranean space came into view — a logistics hub, hundreds of yards long and at least three yards high. Connected to it was a buried web of pipes that once carried natural gas between Ukraine and Russia.

Ukrainian troops cannot stand up in the pipes, which are less than four feet in diameter. Yet they spend weeks at a time inside them, fighting Russians with guns and grenades, ferrying out fellow soldiers injured or killed at the front, and carrying in critical supplies. With drones stalking the battlefield above, this is the future of war, Ukrainian strategists say.

As I approached one of the pipes, the smell of residual gas made my head spin. Soldiers caked in brown dust constantly popped out from or disappeared into the tunnel on the rusty electric buggies that roll through the underground network.

I put on protective goggles and a gas mask and sat behind troops who had been ordered to their positions. Inside the pipe, the smell would be even stronger, they said.

Then we set off.

Riding In

It hadn’t taken long on this October day to see why the Ukrainian military wants to take more of the battle against Russia below ground.

To get to the hole in the earth that led to the pipe, our photographer, Finbarr O’Reilly, and I first had to cross open fields in a pickup truck with a group that included three soldiers. We were within range of drones that target anything that moves.

The soldiers were quiet, the mood tense. One of them held a drone detector in one hand and a phone in the other, asking reconnaissance pilots monitoring the stretch of road about “dots in the sky.” The detector kept beeping.