How do you map what you can't see?

Have you ever used a map to find your way to a new location? If so, you can appreciate the work it takes to create a map that shows you exactly where everything is and how far away it is.

Maps not only show us where things are, they can also show us the kinds of terrain in each area - hills, mountains, lakes, and rivers. Maps that show us such surface features are called topographic maps. Other maps show us even more. They show what lies under the earth. These are called geologic maps.

Geologic maps can help you find your way across the landscape, but they have many other important uses, too. They help locate fossil fuels, such as natural gas and oil, as well as minerals and underground water supplies. They help builders locate safe, secure sites for new buildings and guide engineers planning the route of a new highway.

They also preserve the environment, because they can help predict and measure erosion along streams and beaches. Knowing the types of soil and rock beneath the surface helps environmentalists predict the effects of new construction or figure out why an area's water or other resources are changing. Engineers use geologic maps to help them plan the safest places for dams.

An understanding of what's underground has helped businesses and institutions save energy and money. For example, the Utah State Prison is located directly over a geothermal aquifer, an area where water below the surface is naturally warmed by hot rock.

The 185-degree F. water is pumped to the surface from about 1,000 feet underground. The water provides hot water and heating for employees and inmates. The water is pumped to a cooling pond before flowing into local rivers. State planners hope to turn the cooling pond into a wetland habitat for local wildlife. Using this geothermal energy source saves hundreds of thousands of dollars each year. It also eliminates the pollution that would be created by using another energy source.

Geologic maps also help us determine areas where earthquakes might occur or where volcanoes could erupt. Maps of the ocean floor can help us track where tidal waves might occur when an earthquake or landslide happens under the ocean. As recent events have shown, tidal waves can do a lot of damage. But if an area is prepared and warned when a tidal wave is likely, people can move away from the coastline until the danger is past.

One more use for geologic mapping is to unravel the story of the earth. Studying the way rocks and soil have shifted, and the layers below the surface, give us clues to how the earth has changed over hundreds of millions of years. By studying geologic formations, scientists can discover how mountains formed, continents collided, and oceans appeared - and disappeared.

So how are geologic maps made? Topographic maps are made by looking at the surface, but geologic maps require some detective work. Geologists often begin with aerial photos. They take pictures of the area from an airplane. By putting together two pictures of the same area taken from slightly different positions, they can create a three-dimensional image of the ground. Satellite photos can help identify large surface features and deposits of sediments and rocks.

Geologists then explore the area on foot, examining rocks and sediments to figure out what might be underneath. Some outcroppings may show evidence that they have been folded or separated during earthquakes. If a portion of the ground has been pushed up, it may reveal the layers of rock and sediment below.

Drilling may provide a peek at what lies underfoot. A hollow pipe pushed deep below the surface can bring up a tube of dirt and rock called a core sample that contains all the different layers intact for study. In areas where oil or gas companies have drilled deep into the ground, samples can also be brought up through their drilling holes.

Seismic studies also help. A seismometer is used to measure movements of the earth. By measuring the way different parts of the earth move during earthquakes, scientists can calculate how solid the different layers of earth are and what types of rock might be there. Earthquakes happen every day, but most are so small that we don't feel them. Instruments measure them, though, and the data helps us see inside the planet.

To see where earthquakes are occurring in the United States, go to the US Geological Survey website at: earthquake.usgs.gov/recenteqs. It has a map locating the earthquakes for the past seven days. You may see hundreds of earthquakes on the map, most of them unnoticed by humans.

You can make a seismometer to measure movements in your home at: pbskids.org/zoom/activities/sci/seismometer.html.

Geologists at the Illinois State Geological Survey in Champaign, Ill., have been using Shallow Seismic Reflection (SSR) to map the geology of their state. Special microphones that detect sound waves are mounted on a string of sleds and are dragged behind a vehicle. This system has been used to find the most stable routes for new roads and to identify underground water sources.

Geologists take information from all these sources and use computers to put together a geologic map. It may take months, even years, to complete a map of one area. Today, scientists have a general picture of what makes up the earth, right to its very center. But determining the detailed structure in a particular area takes more work. We now have detailed maps to a depth of about 10,000 feet in the areas that have been most heavily studied.

In 1992, the US Congress passed the National Geologic Mapping Act to help support states in their geologic mapping efforts. The United States Geologic Survey website is now building a National Geologic Map Database (ngmdb.usgs.gov) so geologists across the country can organize their geologic information in a single resource. Scientists, developers, environmentalists, and others can access more than 70,000 geologic maps and other data at this site. Geologists in every state continue to add new information to help us understand what lies beneath.

The earth isn't the only place geologists map. They have examined other planets and moons and tried to figure out, from a long distance, their geologic structure. When the United States first decided to send astronauts to the moon, geologists were thrilled with the idea of examining actual moon rocks. They also found ways to 'look' below the lunar surface.

When the first humans set foot on the moon, they brought seismometers to measure movements of its surface. Apollo flights 11, 12, 14, 15, and 16 placed seismometers on the moon.

To calibrate these instruments, NASA created a few 'moonquakes.' After the astronaut crews had resumed orbiting the moon, they sent their lunar landing vehicles crashing onto the moon's surface. The seismometers measured the resulting vibrations.

More moonquakes were created by the spent third stages of the rockets that had lifted the Apollo spaceships from Earth. The rocket stages were separated from the command modules (where the astronauts were) and sent hurtling into the moon. The seismometers also measured natural moonquakes caused by the impact of more than 1,700 meteoroids. The geologic map below was assembled partly from data relayed by the lunar seismometers.

You can learn more about these experiments and their results at the website: www.lpi.usra.edu/expmoon/ Apollo11/A11_Experiments_PSE.html.