In Mark Twain’s “The Adventures of Tom Sawyer” (1876), Tom and pal Becky get lost in McDougal’s Cave, leaving the residents of St. Petersburg, Mo., to frantically search for the children underground. Townsfolk fear for Tom’s and Becky’s lives. After all, it’s dark, they’re hungry, and Tom’s nemesis Injun Joe is about.

Good that Twain was writing in the 19th century. These days, this plot point wouldn’t have been worth much — St. Petersburg could have simply deployed a spelunking drone to find the lost kids. Half a world away, that’s what an avid spelunker just used to produce this startling video of Hang Son Doong in Vietnam, billed as the largest cave in the world.

“Caves are one of the last unexplored places in the world,” Ryan Deboodt, 31, told The Washington Post in a telephone interview from Beijing, where he holds down a day job running a rock-climbing wall at a school. “…  It’s an alien world. I’ve always wanted to explore the planets. That’s obviously not possible, so this is the second-best thing.”

Though Twain didn’t specify exactly how big McDougal’s Cave was, Hang Son Doong probably dwarfs it. At three miles long, 660 feet high and 490 feet wide, this is less a sightseeing stop than a unique environment. As Discover put it: “Just calling Hang Son Doong a cave almost does it a disservice. With its own river, microclimate and jungle, Hang Son Doong is more than a cave; it’s a world unto itself.”

Deboodt, who had taken time-lapse images of Hang Son Doong before, focused on the cave’s dolines — sinkholes that, for the spelunker, become skylights — during a January visit with the drone. He said the cave had been threatened by a cable car — a project since put on hold, according to Caving News — slated to run right through it, and he wanted to document the environment before it disappeared.

We would cross underground rivers, see what are probably the largest stalagmites in the world (87 yards high), see 300 million year old fossils embedded in the limestone, and witness some of the greatest and most unique views on this planet,” Deboodt wrote on his blog.  

Here’s a map of the cave for anyone interested in taking a trip. Don’t forget the trail of breadcrumbs.