If you've ever visited the Chesapeake Bay -- to sail or have a picnic at Sandy Point State Park -- the blue water probably seemed beautiful.

But below its sparkling surface, the Chesapeake is becoming increasingly unhealthy. For the plants and animals that live there, things are not so beautiful:

* Oysters are almost gone from the Chesapeake. The yearly catch fell from 1 million bushels in 1990 to 26,500 last year.

* Crabs also are at an all-time low. In just one year, 2003, the bay lost about a third of its underwater grasses, the only place where baby crabs and little fish can thrive.

Now there is a "dead zone" in the bay that in the summer stretches for hundreds of square miles, scientists say. In that zone, there is so little oxygen in the water that plants and animals can't survive. (Picture your fish tank if the filter stopped working!)

What has happened to the Chesapeake Bay, America's largest estuary? (An estuary is a coastal area where fresh water and salt water mix.) What could produce enough pollution to mess up 18 trillion gallons of water?

To understand the Chesapeake's troubles, you have to know that six major rivers and thousands of small creeks (in six states and the District) flow into it. This area is known as the bay's watershed.

Chicken and cow poop, trash, fertilizer from farms and lawns, human waste that hasn't been properly treated, dirt that washes away from construction sites -- all make their way along those rivers and creeks into the bay. That causes big problems, scientists say.

About 16 million people live in this area. That's double the population of 50 years ago. And the number is expected to keep growing. All those people have to live somewhere. So the challenge -- for scientists, politicians, farmers and everyone else -- is to figure out how all of us can live, eat, shop, flush and get to school and work without destroying the bay.

People who have begun tackling the problem know that it's tough. Fixing aging sewage systems costs billions of dollars. Passing laws or spending big bucks to get farmers to switch to less-polluting ways isn't an easy answer either.

Recently, 16 students from Turner Ashby High School in Rockingham County, Virginia, took a 30-day, 350-mile canoe-and-kayak trip through the Potomac River Basin to better understand how people are hurting -- and helping -- the bay.

The teens shared with KidsPost what they saw, heard -- and smelled -- along the way. Trip leader Kelsey Brunton, 17, advises kids to be aware "that the bay needs help and that everything you do affects it."

-- Fern Shen

Trash and water pollution were among the topics Lamar Thomas of the Anacostia Conservation Corps discussed with Tara McFarland and other students taking a 30-day, 350-mile journey through the Potomac River Basin to the Chesapeake Bay.From top: a pumpkinseed sunfish with a large lesion, a sore often caused by too many nutrients in the water; sewage outflow from a plant in Shenandoah, Virginia.From top: Curt Newsome and Jordan Diehl carry a canoe; excess nitrogen helps create thick algae mats that kill bay life; Gretchen Hale tests the water.