There is no more intimate and eerie way to explore this man-made wonder than hovering above it and floating through its guard towers.

More than 50,000 people visit the Great Wall of China each day during the high season; fewer than 50 people a year do it underwater. At a little-known section that is submerged beneath the 26-square-mile Panjiakou Reservoir (the area was intentionally flooded in 1978 to provide drinking water to the region), it's possible to don scuba gear and explore all three sides, top to bottom, while hovering weightlessly.

Our guide is New Jersey-born, Beijing-based divemaster Steven Schwankert – Indiana Jones in a wetsuit, who says he loves "offbeat" diving and specializes in trips to places like Siberia's Lake Baikal for ice diving. "Divers are always saying, 'Take me to something I haven't seen before,' " he explains. "That's why I came here."

Underwater, the Great Wall is an eerie Chinese Atlantis – its hulk of bricks seems trapped in time as you drift above 100-foot parapets and float through tunnels all by yourself. Your first chance to touch the stone comes at just 12 feet deep. Like exploring a sunken ship, there's something strange about swimming around a place that normally sits above the surface. In its entirety, the wall is longer than the width of the United States. Diving it in a dark algal haze, examining it in the light cast by an underwater flashlight, offers a bizarrely microscopic perspective. We examined it from just inches away, running my hands along the ancient stonework. At 60 feet, the underwater flashlights come on and the mortar between the bricks shines as slivers of reflective white. At 91 feet, there is a 15-foot-thick guard tower; swimming through it is like floating through a giant tunnel. The ultimate goal is a "sally port" at 105 feet. A thousand years ago, these openings allowed defending troops to move from side to side, and there are very few of them anywhere on the wall.

Near the surface, the water is practically Caribbean-like. Down past 100 feet, it drops to 42 degrees. Visibility was also getting worse, bringing me even closer to the wall. We were careful to keep our fins off the bottom, or we'd kick up enough crud that we wouldn't be able to see anything at all.

Our bodies were going numb and we could barely see a thing, but we still couldn't help but think about something Schwankert said before we jumped in the water: "It's so up close and personal, so amazing to focus on the stones and bricks that it hardly makes sense to walk it in the open air anymore."

More Information: Fly to Beijing; the reservoir is a 3.5-hour drive north. Overnight dive trips, which include three dives, meals, accommodation, and basic equipment, cost $610. Full scuba certification, taught in English, is available. [$240; sinoscuba.com]