To you， a rock is just a rock - something you can study， or climb， or throw into a pond. But to bacteria and other tiny organisms that live deep underground， rocks and minerals may offer something extra： The energy for staying alive.

Microscopic organisms， often called microbes， are so small you can usually see them only under the microscope. Microbes living in rock beneath Earth's surface seem to be able to secure their own food. When microbes are mixed with ground-up minerals and heated， the minerals produce hydrogen. The microbes eat the hydrogen and stay alive. When the microbes aren't around， the minerals produce barely any hydrogen.

The bacteria appear to help get the hydrogen， said a geomicrobiologist， who led the new study of microbes and minerals. Geomicrobiologists study how microbial life interacts with rocks and minerals when microbes live deep underground.

In the last few decades， scientists have had to change the way they think about how microbes live on and beneath Earth's surface. It turns out that microbes have been discovered living in harsh underground environments， where the heat and pressure would quickly extinguish human life.

Scientists now estimate that twice as many microbes live underground as those live on the surface or close to it. It may answer questions about how life can get energy and exist miles underground， where the Sun doesn't shine. Without sunlight， organisms cannot use photosynthesis to produce food， so they must feed some other way.

Earlier experiments have shown that microbes living deep underground feed on organic matter that settles on the ocean floor. Organic matter comes from organisms that used to be alive， including plants and animals. But for their experiments， scientists decided to see whether inorganic matter - stuff that was never alive， such as minerals - could also provide fuel.

The researchers first ground up minerals， like quartz and basalt， and mixed in some organic matter. Into some samples， they also added microbes. The mixtures were heated， and those that contained microbes began producing hydrogen as the temperature climbed. The mixtures without microbes， which had been sterilized to kill any living things that might have snuck in， produced very little hydrogen. It suggests that the minerals couldn't make hydrogen unless some microbes were hanging out.

1. It is estimated that ____.

A. more microbes live on the surface of the earth than underground

B. more microbes live on the seafloor than on land

C. more microbes live underground than on the surface of the earth

D. microbes on the surface of the earth are twice as many as those living underground

2. Which is the necessary element for photosyn-

thesis to happen？ ____.

A. quartz B. basalt C. sunlight D. minerals

3. What is the job of a geomicrobiologist？ ____.

A. To study life and the earth

B. To study animals and plants

C. To study how the earth has changed

D. To study microbes