The small group is experiencing Battle Creek Cypress Swamp at its swampiest. Spring rain is plopping into the limpid pools, marring the mirror image of the bald cypress trees that merge into a cathedral ceiling 150 feet overhead. Sneakers squeak on the wed boardwalk that naturalist Dwight Williams built through the swamp, which is owned by the Nature Conservancy. Battle Creek, the small stream which feeds the swamp, gurgles with rainpower. And the cypruss "knees," cinnamon-colored roots that rise four feet out of the water, are shiny-wet.

The rambling roots or knees are one of nature's riddles. Scientists once believed they were snorkles that help the trees breathe. Now the prevailing theory is that the knees help stabilize the tree in its boggy foundation.Even the fact that the swamp is where it is -- in Calvert County, Maryland -- is a bit of a mystery. Bald cypress stands are common in the south, but rare this far north and the site is a national historic landmark.

The bald cypress is the only conifer that sheds its foliage -- tiny, lacy leaves and the branchlets that hold them. The canopy of leaves closes over in May, Williams says, but even without it the visitor can spot no birds but only listen to the shrill, staccato call of the tufted titmouse. At least that's what Williams thinks it was when it's described to him back at the nature center that's at the beginning and end of the quarter-mile loop.

"Warblers are the main migrants we get here," he adds, " and there are a lot of pilated woodpeckers. By May, there'll be wildflowers around the trail, especially violets, spring beauties and bloodroot."

In the recently opened nature center, which smells of new wood, there are catfish in tanks, snakeskine and microscopes to look at them with, and games where if you press the button that correctly identifies a bird or animal a buzzer sounds.

Another question for the naturalist: What was that gray growth on the cypress bark?

"Did you touch it? Did it feel like a Brillo pad?" Williams asks.

When the answer is yes to both, he smiles ruefully.

"I guess we ought to have some sort of interpretive sign about that," he says. "That was poison ivy."