Like explorers of old, scientists are venturing into the immense but little-known realm of the microscopic organisms that dominate our planet.

“It’s an entire world that most of us have no idea about,” said Alan Leshner, the chief executive of the American Association for the Advancement of Science.

Countless trillions of microbes — mostly bacteria and a recently discovered kingdom of one-celled creatures known as archaea — inhabit every cranny of the globe. They reshape their environment, make life possible and sometimes destroy it.