By Mark Kennedy, AP Entertainment Writer

Just in time for Halloween comes a new documentary focusing on one of Earth’s most frightening treats — volcanos. Even better, swashbuckling director and writer Werner Herzog is the filmmaker, so it’s sure to be a hair-singing descent into the fiery heart of volcanos.

But, alas, Herzog has really just pulled a nasty trick on us. The legendary filmmaker’s “Into the Inferno “ — not to be confused with the new Tom Hanks film “Inferno” — is actually a lazy, meandering mess that gives off no heat.

Herzog takes us to steaming, scary volcanic mountains in North Korea, Indonesia, Iceland, Ethiopia and the Pacific island nation of Vanuatu but never really connects them thematically or narratively. Often the volcanos are studiously ignored.

In North Korea, Herzog spends a lot of time discussing that closed society’s intriguing use of propaganda. In Ethiopia, he follows archeologists. And in Indonesia, we have an excruciating discussion with volcano tech monitors about electronic distance measurements and gas emissions.

These visits are interspersed by what could best be described as volcanic porn — nighttime shots of gorgeous lava as it oozes down a mountain or bursts like cheese bubbling on a cooking pizza, all set to opera or music by Verdi, Rachmaninov and Vivaldi.

“It is hard to take your eyes off the fire that burns deep under our feet,” Herzog intones in his dreamy German accent during such a sequence. “Everywhere — under the crust of the continents and sea beds. It is a fire that wants to burst forth and it could not care less about what we are doing up here.”

That’s promising stuff but then we’re off to another volcano and another pointless look at some feature of that country. In one — the barren, scorching Afar Region in Ethiopia — Herzog films a crew of archeologists hunting for remains of Homo sapiens. We watch a Western fossil hunter sift through dust for tiny fragments. “Are we ready to rock and roll? Let’s get brushes,” he says.

What does this have to do with volcanos? Very little. What does his investigation on Vanuatu about the cult surrounding the mythical figure of John Frum have to do with lava? Not much. The film pretends to be investigating indigenous spiritual practices around volcanos but simply never delivers. Instead of rock and roll, we get the brush off.

We even learn that a lot of “Into the Inferno” lifts from a couple of Herzog’s other films — “La Soufrière,” which looked at volcanic activity on Guadeloupe, and “Encounters at the End of the World,” about Antarctica.

It was in that latter documentary that Herzog met and befriended Clive Oppenheimer, a volcanologist from Cambridge University who acts as interviewer in the new film and whose book, “Eruptions That Shook the World,” inspired it.

This mix of old and new, and the mingling of friendship and filmmaking may be why “Into the Inferno” lacks focus. It sometimes seems like Herzog did a bait-and-switch — promising to make a volcano film and really just using that as an excuse to travel to some cool places with a pal.

That’s fine, but the documentary he made is unwatchable. The Tom Hanks film is probably a lot better.

“Into the Inferno,” a Netflix Documentary, is rated TV-PG. Running time: 1 hour, 47 minutes. No stars out of four.

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