At 3 o'clock on Sept. 4, 1882, the electrical age began. The Edison Illuminating Co. switched on its power plant, and a network of copper wires came alive, delivering current to a few dozen buildings.

But nature invented the electrical grid first, it turns out. Even in 1882, thousands of miles of wires were already installed in meadows, in salt marshes, in muddy river bottoms. They were built by microbes, which used them to shuttle electricity.

Electroactive bacteria were unknown to science until a couple of decades ago. But now scientists are finding this natural electricity across much of the world, even on the ocean floor. It alters entire ecosystems, and may help control Earth's chemistry.

"Not to sound too crazy, but we have an electric planet," said John Stolz, a microbiologist at Duquesne University.

In the mid-1980s, Stolz was helping to study a baffling microbe fished out of the Potomac River by his colleague Derek Lovley, now a microbiologist at the University of Massachusetts at Amherst. Like us, Geobacter feed on carbon compounds. As our cells break down these compounds to generate energy, they strip off free electrons and transfer them to oxygen atoms, producing water molecules.

Geobacter couldn't use oxygen. Instead, it transfers its electrons to iron oxide, or rust, Lovley and his colleagues discovered. The process helps turn rust into another iron compound, called magnetite.

Humans draw oxygen into our cells to use it, but Geobacter does not import rust. So the microbe must get electrons to attach to rust particles. How?

In the early 2000s, Lovley's team discovered that Geobacter could sense rust, responding by sprouting hairlike growths. Maybe each of those growths, known as a pilus, was a wire that latched onto the rust, Lovley thought. Electrons could flow from the bacterium down the wire to the receptive rust. "It seemed like a wild idea," he said.

But he and his team found several clues suggesting that the pilus is indeed a living wire. In one experiment, when Geobacter was prevented from making pili, the bacteria couldn't turn rust to magnetite. In another, Lovley and his colleagues plucked pili from the bacteria and touched them with an electrified probe. The current swiftly shot down the length of the hairs.

In the early 2000s, Danish microbiologist Lars Peter Nielsen discovered a different way to build a microbial wire. He dug up mud from the Bay of Aarhus and put probes in it. He observed the chemical reactions carried out by its microbes. "It developed in a very weird direction," he said.

At the base of the mud, Nielsen observed a buildup of hydrogen sulfide. That alone was not surprising — microbes in oxygen-free depths can produce huge amounts of hydrogen sulfide.

But the hydrogen sulfide in the Aarhus mud never made it to the surface. Something was destroying it along the way.

Nielsen woke up one night with an idea: "I imagined it could be electric wires." So he and his colleagues looked for wires, and they found them. But the wires in the Aarhus mud were unlike anything previously discovered.

Each wire runs up through the mud, measuring up to 2 inches in length. And each one is made up of thousands of cells stacked on top of each other. The cells build a protein sleeve around themselves that conducts electricity.

As the bacteria at the bottom break down hydrogen sulfide, they release electrons, which flow upward along the "cable bacteria" to the surface. There, other bacteria use the electrons to combine oxygen and hydrogen and make water.

Cable bacteria are not unique to Aarhus. Researchers have found them — at least six species — in many places around the world.

Electroactive microbes are so abundant, in fact, that researchers now suspect that they have a profound impact on the planet. The bioelectric currents may convert minerals, fostering the growth of a diversity of other species. Some researchers have speculated that electroactive microbes may help regulate the chemistry of both the oceans and the atmosphere.

"To me, it's a strong reminder of how ready we are to ignore things we cannot imagine," Nielsen said.