Then there’s that pesky missing exit event. A primordial black hole should’ve sliced its way straight through the earth and out the other side, causing near as much destruction going out as it did going in. But Bill Beasley and Brian Tinsley went back and studied the barometric readings for June 30th, 1908, and there’s just no trace of the thing’s erupting up out of the earth on its way back into space. Of the two objections, this one hurt the more, since Al and Mike had gone and staked their whole hypothesis on finding evidence of an exit explosion.

When you look at them up close, though, you find these two counterarguments are riding, not so much on facts, as on assumptions. Assumptions that might’ve seemed pretty solid three decades ago, but maybe haven’t stood the test of time quite so well as we’d like.

What I’m trying to say is, it might only take a few tweaks — a few ideas from more recent black-hole physics — to turn Jackson-Ryan back into a viable hypothesis again.

Now, bear in mind, I’m not going to be talking much about being able to prove Jackson-Ryan here. It’s way too early for that. Yes, we did look at some possible evidence for a revised Jackson-Ryan hypothesis the last time (principally, the Irkutsk Observatory magnetograms and the Kiel University compass readings), and we’ll look at a little more below. But, let’s face facts: the cometarians and the meteoritists have pretty much had the evidence-gathering game to themselves for the past three-quarters of a century. Every expedition to Tunguska mounted since 1927 has gone out there with Tom Gehrels’ mantra about how “Scientists have always understood that it was a comet or asteroid” tattooed to their foreheads. And if that’s all you’re looking for in a Tunguska Object, well — surprise, surprise! — that’s all you’re going to find.

Except they haven’t found it. Not in all that time.

Maybe it’s time to look for something else.

But it’s theory that guides us in what to look for. So, it’s a change in the theory — what Tom Kuhn called a “paradigm shift” — that’s pretty much got to come first. There’s no way to put the evidentiary cart before this particular hypothe-horse. Without a theoretical framework to show how and why it’s significant, what might otherwise be seen as promising evidence can wind up overlooked, dismissed as “observational error,” or, as in the case of magnetic storms and compass deviations, just plain forgotten.

So, all I’m looking to do here is fix the theory, at least enough that the researchers will start looking for the evidence that’ll test it, one way or the other.

The tweaks to Jackson-Ryan that I’ve got in mind aren’t all that major, either. Mostly they take the form of rethinking what kind of a primordial black hole might’ve caused the Tunguska Event. Because any PBH that could overcome the thermal-seismic and exit-event objections would have to be different from the kind Al Jackson and Mike Ryan had in mind. Different enough to need a whole new name.

I’ve been calling it a “Vurdalak” here, after the Russian word for “vampire” or “werewolf.” (Happy Halloween!)

If that makes it sound dangerous, it’s supposed to. Because this new kind of micro-black hole could be a lot more destructive than the old one. In the long run, anyway.

So, what’s this “Vurdalak” black hole like?

Well, for one thing, it’s a lot less hefty than the one Al and Mike were working with. They’d figured on their black hole weighing in at anywhere from a hundred trillion metric tonnes on up — the mass of a small asteroid. And Burns, Greenstein, and Verosub used basically the same numbers to come up with their estimated thermal and seismic effects.

But where’d Al and Mike get that mass estimate to begin with? Simple: they’d started with the force of the explosion itself and worked backwards. As Al Jackson told me, “the mass came only from the shock wave energy.”

Now, what with the devastation at the impact zone, not to mention the pressure wave recorded on barographs around the world, figuring the explosive force wasn’t all that hard. Most estimates come out pretty consistent, falling into a range somewhere between two and forty megatons.

But, and here’s the point, when Al and Mike worked the numbers backwards, they did it by assuming all that energy had to come from gravitational effects alone. That made sense at the time, because, aside from its mass, a 1973-vintage black hole had no other way to interact with its environment.

Look, it’s pretty easy for a “normal” meteorite or comet fragment — basically just a big chunk of iron or ice — to release a lot of energy on the trip down, by ramming the air out of its way and piling it up into a shockfront. But a micro-hole’s cross-section is smaller than an atom. At most, its flight path isn’t going to intersect all that many air molecules, and the few it does run into don’t get shoved aside — they get eaten.

So, if our PBH had nothing but mass going for it, that mass would’ve had to have been huge. So huge, in fact, that it would’ve also had to generate all those thermal and seismic side-effects — the miles of fused rock, the mantle-busting earthquakes — which, by their absence, supposably disproved the Jackson-Ryan theory.

Now, along comes Vurdalak. Compared to Al and Mike’s original asteroid-sized PBH, he’s just a little guy. Thousands, tens of thousands of times smaller. No more than five or ten billion tons tops, the mass of a small mountain maybe, and the size of an atomic nucleus. Just an order of magnitude or so above the limit for spontaneous, catastrophic total evaporation, in fact.

And, not incidentally, way too small to have caused anything like the effects that Burns and company predicted.

But how could anything that small (if you can call five billion tons “small”) have produced a forty megaton blast, not to mention the sound- and lightshow leading up to it? Well, unlike the old-model PBH, our Vurdalak’s got something more than mass going for it, something nobody’d even guessed at back in the early 1970s: Hawking radiation.

And that radiation increases as the hole gets smaller, remember? Teensy as our Vurdalak is, it’d have a temperature somewhere in the billions of degrees. ’Course, it’s only that hot right at the event horizon, which is no bigger than the nucleus of an atom. Move even a little distance away, and the inverse-square law brings the heat down fast. At a meter or so out, for instance, it wouldn’t be a whole lot hotter than your average blast furnace. So it’s not like Vurdalak’s going to scald half the planet coming in. Still, it would be pumping out photons of all kinds, from visible light on through the ultraviolet and all the way up to x-rays.

The atmosphere’s opaque to those shorter wavelengths. (That’s bad news for astronomers, good news for the rest of us, seeing as how it keeps the incidence of skin cancer down.) The gases that make up the air will stop UV radiation, for one, right in its tracks.[1] But, in the process, the nitrogen and oxygen and other molecules that do the actual blocking’ll have their electrons stripped clean off.

That turns those ordinary, neutral molecules into ions. Charged particles, that is. A lot of charged particles. On final approach, Vurdalak’s ionizing radiation would’ve laid down a contrail that’d put an Airbus-II to shame.

And that alone’s enough to give us the visuals: a bright blue tube that splits the sky in half, just like the eyewitnesses said. Think the Fourth of July, Bastille Day, George Lucas, and a small nuclear war, all rolled into one!

Now, pile on another assumption: that Vurdalak’s a “black monopole” — not just a primordial black hole, but a magnetic monopole besides. Then picture how the lines of magnetic force radiate straight out from it in all directions, like spines on a Koosh-ball. And picture that black monopole trying to plow its way through hundreds of miles of atmosphere ionized by its own radiation.

That’s going to have an effect! Being charged particles, the ions forming to Vurdalak’s front and sides aren’t going to want to cross those radial lines of magnetic force it’s putting out. Instead, they’re going to “stick” to the hole and get dragged along for the ride. And because Vurdalak is traveling much faster than the speed of sound, they’re going to have to break the sound barrier too.

And there’s your sound effects: continuous sonic booms all along the flight-path. Fourteen or more bangs on up, according to some eyewitnesses. Observers on the ground would hear and feel an extremely complex jumble of sound, maybe even further scrambled by refraction through the atmosphere’s temperature gradients. You’d get, in other words, pretty much what eyewitness T. N. Naumenko described: A series of thunderclaps, followed by —

one so strong it was as if it had several crashes mingled together inside of it, with such a crash that the ground shook, and throughout the taiga there reverberated such an echo, and not even an echo, but some sort of deafening solid roar. It seemed that that roar enveloped the whole taiga of unencompassable Siberia.

But what about that forty-megaton explosion? Seems like, if Vurdalak was too small to have produced Burns and company’s thermal and seismic effects, it’d also have been way too small to kick up much of a fuss when it hit. It’s for sure it couldn’t’ve flattened and burned all the trees — 80 million of them according to the latest estimates! — across an area half the size of Rhode Island. It couldn’t’ve burned the shirt off the back of a man standing forty miles away. It couldn’t do all that, now could it?

Turns out it could. Because, remember: Vurdalak’s dragging all that ionized air along with it, sweeping up more and more of it the further it goes. Hundreds of thousands, maybe millions of tons of it, by the time the hole traverses the four, five hundred miles of atmosphere along its shallow-angled approach path.

Now, imagine what happens when Vurdalak hits. By itself, it’s not going to have much effect outside a couple meters radius. But then this faster- than-sound hurricane comes hammering down behind it. A mountain of air piles up, compresses, heats. A gale-force superhot wind blows outward in all directions from the epicenter, flash-burning the bark off the trees even before they have time to fall, leaving nothing but smoldering ruination behind.

So it seems like we can produce all the phenomena of the Tunguska Event using a much smaller hole than Al Jackson and Mike Ryan ever dreamed of, long as it’s a magnetic monopole besides.

But that’s not all you get for your ruble. Because, tucked away in amongst the mechanics of creating sonic booms, and lightshows, and forest fires, there’s also a solution to the biggest objection of all. The one even Carl Sagan leveled against the Jackson-Ryan hypothesis in that Cosmos episode:

The missing “exit event.”

It’s that miles-long column of air Vurdalak’s pulling along behind it, you see. Like I said , it’s being accelerated to the speed of sound and beyond. And, in physics at least, there’s no such a thing as a free lunch — the energy needed to speed up all those megatonnes of atmosphere has got to come from someplace.

The only place it can come from in this case is Vurdalak itself. Hard as our new-fangled radiating, magnetic PBH is pulling on all those air molecules, they’re pulling back on the PBH just as hard.

What that gives you is magnetic air-braking: The more air Vurdalak drags along in its downward dive, the slower it goes.

Hit those air-brakes hard enough, and old Vurdalak will maybe slow down to where it can’t climb back out of earth’s gravity well anymore.

So, the real answer to the question Where’s the exit event? is another question:

What if it never came out?