Simply put, Hilbertz has found a way to use sunlight to turn the minerals in seawater into limestone — accretion — depositing waterborne solids.
Autopia Ampere will begin as a series of wire-mesh armatures anchored atop a sea mountain. Once in place, they will be connected to a supply of low-voltage direct current produced by solar panels. Over time, electrochemical reactions will draw minerals from the sea to the armatures, creating walls of calcium carbonate, which is what us landlubbers commonly call limestone.
The fact that ocean-grown cities could stand on their own economically and become independent and self-governing entities poses what Hilbertz believes to be one of the biggest barriers to their creation. He says there is no legal precedent regarding national ownership of a newly formed island that is beyond a nation’s territorial waters.
from Popular Mechanics:
Mineral accretion technology was developed originally in the 1970s by architect Wolf Hilbertz to generate alternative construction materials The Army Corps of Engineers has used electrical accretion to build reefs and seawalls.
In 1980, I experimented with building mineral boxes by sending DC current generated by a windmill into submerged metal re-bar and screen. I imagined houseboat-sized barges could be manufactured, and this may become true someday.