A coat of titanium dioxide paint brushed onto the silvery shell will neutralize pollution by absorbing ultraviolet rays, enabling a chemical reaction that decomposes organic and inorganic toxins. (It's the same technology used in certain high-tech concrete that breaks down airborne particulates.) As the vessel whips along, purifying waterways, it can draw on both solar and hydro power. Turbines under the hull transform water movement into electricity, and rooftop photovoltaic cells harness energy from the sun. The roof doubles as a nursery, whose carefully selected plants help filter river gunk, whether from the Thames, Rhine or Euphrates.