Crack open a deep-sea nodule and you’ll find a tiny metal layer cake that grew atom by atom on the ocean floor.

At the very center sits a “seed” — often a grain of sand, or a shard of volcanic glass. Over millions of years, manganese and iron drifting in seawater grabbed hold of that speck and formed thin, dark shells around it.

As each new shell hardened, it trapped other dissolved metals — nickel for battery energy, copper for wiring, cobalt for high-strength alloys, and a dash of rare-earth elements used in phone speakers and wind-turbine magnets.

The result looks like a striped jawbreaker: onion-style layers just fractions of a millimeter thick, but packed with useful chemistry.

Although the average nodule is only six to fifteen centimeters wide, a single one can contain enough nickel to coat hundreds of smartphone batteries and enough manganese to treat several tons of steel.

Multiply that by the billions of nodules carpeting the Clarion–Clipperton Zone and you get one of the richest metal stores left on the planet.

Engineers call nodules “batteries in pebbles” because they bundle the exact cocktail of metals that clean-energy technologies crave.

Unlike land ores that often hold just one main metal, these seafloor pebbles serve up four at once, potentially cutting mining waste.

But grabbing them means sending robot harvesters across sensitive seabeds, so scientists and regulators are debating how to balance the metals we need with the ecosystems we barely understand.

Inside each nodule, then, is not just a mix of elements but a snapshot of the choices society faces as it powers the future.