WHENEVER I VISIT the National Aquarium in Baltimore, it's crowded. I adore standing still at the huge tanks, my eyes fixed on the exotic species, but such contemplation is a luxury. More often, the press of the crowd necessitates a quicker glance than these fascinating creatures deserve. The aquarium's immersion tours -- guided, behind-the-scenes looks at everything from dolphin play to marine animal rescues -- presents one solution.

I recently attended "Sleepover With the Sharks," an overnight program limited to 35 people. Although the tour appeals to both families and adventurous adults, I'd been warned that the evening would be filled with students from Roanoke, so I brought along my own boys, ages 10 and 12.

Sleepover participants arrive at the aquarium at 4:45, just as most visitors are departing. One of our guides, Ed Carlson, gave us $10 vouchers for the cafe and let us drop our sleeping bags in a classroom. We had until 6:15 to wander and eat. Carlson advised us to see the rainforest while it was still light. We headed through blissfully unpopulated corridors to the fourth floor.

Knowing that we were going to spend 16 hours here gave us a real sense of ownership. We felt special as we spied a sloth motionless above our heads. We even had time to read all the exhibit displays along the way.

After dinner, we met our other guide, aquarium instructor Linda Meakes. Our activities began with a game of shark bingo, in which we had to ask one another such questions as, "Have you ever caught a ray?" or "Have you ever touched a shark?" We had to find someone who could answer yes to fill our bingo card.

After this icebreaker, we headed to the dolphin tank for the evening feeding. A calf was born in June, and dolphin shows have been suspended so it can thrive away from the hubbub of the crowds. The atmosphere at the feeding was muted, although clapping broke out whenever a dolphin decided to show off. We glimpsed the fin of the tiny calf swimming alongside its mom, Nani, in one of the rear pools. Large screens replayed a video of the birth.

After watching the dolphins, we split into two groups. One headed for a behind-the-scenes tour, while the other visited the shark lab. Carlson, a geologist by day who also scuba dives inside the aquarium and participates in animal rescues outside, led my group to the food-preparation area.

"Oooh, it smells like fish," cried one of the students' chaperones.

Stepping into an enormous freezer, Carlson held up a "ray tray," an assortment of raw fish for ray consumption. We also took a gander at such delicacies as "seaweed jell" and green "brownies" made of seaweed, both destined for the Atlantic coral reef exhibit. If an animal isn't flourishing, Carlson said, aquarium scuba divers will do a targeted feeding, or hand feeding, of certain items.

Trailing after Carlson, we ducked in and out of doors that most visitors never get to use. We climbed concrete stairs, twisted by huge pipes and around bubbling tanks. In the shark acclimation area, we saw the crane that lifts animals for medical procedures. Nearby, we peered at a small spiny butterfly ray that was hanging out in a tank until it grew big enough to survive in the exhibit. In another room, a baby giant Pacific octopus was suctioned to the side of its tank, also growing in safety. Ordinarily, we would see creatures only through the glass. On this tour, we got to peer down into the tanks as their keepers would. My favorite was the amazingly vivid and varied anemone tank. When we saw them moments later from the public perspective, I was struck by how much brighter their colors were without the glass barrier.

The highlight of the tour was strolling on the shark catwalk. We stepped onto the metal structure and leaned over the rail to admire a huge nurse shark about 10 feet below. Through the shimmering water, I could see each gill and trace the ragged edge of its fin. Its body looked soft and pinkish in the light, although Carlson said we would feel "teeth," or dermal denticles, in their skin if we rubbed the wrong way. The sharks are fed fresh fish from the platform with long poles.

"You can see the sharks don't use much energy as they move," Carlson said. He also reminded us that more people are killed by bee stings each year than by sharks.

Meakes also stressed the rarity of shark attacks. "Very often shark bites are just a case of mistaken identity," she said. Using cutouts, she showed how a surfer on a board looks like a seal.

Later, when we could see the eyes and teeth of the same sharks through the glass, they looked much fiercer than they had from above.

Next, we piled into the Discovery Lab, where we were invited to touch everything. Most were drawn to a sand-filled container in which they would find real sharks' teeth to make necklaces out of. I opted to look at the artifacts -- a sawfish rostrum, consumer products containing shark parts (such as medicinal teas and shark liver oil), dried dogfish skin -- around the lab. The aquarium instructors gear the level of complexity of the activities to their audience. Although I'm an adult, I learned plenty.

After packing our brains with shark facts, we were able to explore the sections of the aquarium we hadn't seen. Everyone else had left, and even the escalators were turned off. We took our time studying the output of an electric eel or watching huge rays from the floors above. Our guides pointed out details that we might have otherwise missed, such as skilletfish eggs laid against the side of the glass. The parents fanned the eggs to keep water running over them. At the "Open Ocean" exhibit, we hung out and stared at the sharks, especially the lovely zebra shark, which has cheetah-like spots and stripes.

Back in the classroom, we had a bedtime snack and watched a documentary highlighting the dangers sharks face. The film was narrated by "Jaws" author Peter Benchley, who states, "Sharks are not the man-eating monsters we have portrayed them to be." It was after 11 by then, so we lugged our sleeping bags down to an open area. (Most groups sleep in the underwater viewing area near the sharks and rays; because of construction, we had to sleep elsewhere.)

Our guides awoke us about 6:30 a.m. After a continental breakfast, Carlson told us that baby seahorses had been born while we slept. Each seahorse pair has about a thousand babies every two weeks. Our noses pressed to the cylindrical tank, we saw hundreds of minuscule, translucent bodies with dark dots for eyes and a tiny curl of a tail. Carlson filled us in on details about seahorse mating dances, camouflage habits and birthing.

Our sleepover ended with a "Jeopardy!"-style game to test our retention of shark facts. About 9 a.m., we shouldered our bags and headed out, just as the daytime crowds began to trickle back. We smiled sleepily at the well-rested folks who were about to enter "our" aquarium.

NATIONAL AQUARIUM -- 501 E. Pratt St., Baltimore. 410-576-3800. www.aqua.org. Sleepovers each month, more often in the summer. The next "Sleepover With the Sharks" events are Dec. 9 and Dec. 26, and both have some openings. $69 for ages 12 and older, $59 for ages 8 to 11; includes admission, dinner and breakfast. Minimum age is 8. The aquarium also offers daytime immersion tours, including the "Dolphin Play" tour, a "Dolphin Discovery" tour, a gallery tour, MARP (Marine Animal Rescue Program) to the Rescue tour and a "Sharks! Behind-the-Scenes" tour. For dates, visit www.aqua.org/immersiontours.html. The 2006 dates and prices have not been announced. Book early because they fill up. For reservations, call 410-576-3833 or e-mail reserve@aqua.org.

Harrison McNabb, left, Daniel Feivor and Barbara Feivor examine anemones at the National Aquarium in Baltimore.Instead of peering at the sharks through glass, participants at the aquarium's sleepover get to see them from a catwalk above the tanks.