All along North Korea’s west coast, the machines rumble to a start. House-sized excavators dig into the ground and shovel earth and rocks into waiting dump trucks. The vehicles drive out on narrow strips of road that extend out from the shoreline, emptying their contents and adding bit by bit to miles-long dikes that — if everything goes to plan — will one day impound the tides and wring dry, arable land out of the sea.

This work is part of a massive state initiative to expand the dimensions of North Korea by more than 1,100 square miles, predominantly