This photo, taken by photographer Nikki Fox, shows cave explorer Dan Austin during a recent underground survey of Jewel Cave.

No one knows for sure just how long the caverns and passageways are in Jewel Cave National Monument. But the known reaches of the world’s second-longest cave recently became a little bit longer.

On April 20, cavers mapped another 700 feet of the underground maze, bringing the total known length to 145.02 miles.

The key word there is “known.” Measurements of the air flow in and out of the cave indicate that just a fraction of the total underground space has been explored. “There’s a lot of cave down there,” said one National Park Service ranger.

(Of course, you don’t need to be a spelunker to explore Jewel Cave. The National Park Service offers daily tours — safe, well-lighted daily tours.)

Jewel Cave, 13 miles west of Custer in the Black Hills of South Dakota, is in little danger of losing its No. 2 ranking. It’s already 10 miles longer than the world’s third longest cave, Optymistychna Cave in the Ukraine.

By the way, the world’s longest cave is 365-mile Mammoth Cave in Kentucky — not to be confused with the Mammoth Site, in Hot Springs, S.D.

And Wind Cave, just north of Hot Springs, has been mapped out at 130 miles, making it the world’s fourth longest cave.

Mapping Jewel Cave’s passageways is an ongoing process. The mapping is done by park staff and by private groups such as the Paha Sapa Grotto, a Black Hills chapter of the National Speleological Society.

Using devices such as compasses and clinometers, which measure the angle of incline, the spelunkers venture to the cave’s unexplored regions. Overall, Jewel Cave stretches about 31/2 square miles, and has at least four levels of mazes.

Ranger Chris Thibodaux said some of the unexplored regions are relatively close to the entrance of Jewel Cave. Previous teams have made note of side passageways as they proceed through the cave. New teams use the old field notes to explore those passageways.

But other unexplored regions are deep, deep underground. There’s a very large room in a very remote location, Thibodaux said. An exploration team will spend eight to 10 hours to reach it. Once they get there, the explorers use it as a base camp. From there, they can spend four hours or so mapping new passages.

Thibodaux doesn’t completely buy the estimate, based on air flow measurements, that just 3 percent of Jewel Cave has been explored. He said that doesn’t mean the remaining 97 percent of Jewel Cave is made up passable passages. It could be tiny cracks, crevices and even pourous limestone.

Just how long could it be? It’s a mystery that we’ll never solve — my favorite kind.