Trouble Wasn’t Long in Coming!
Last time, you’ll recall, we were taking a look at a theory thought up by a couple of good old boys named Al Jackson and Mike Ryan — a theory as to how the 1908 Tunguska Event might have been caused by the earth colliding with a tiny black hole. And we closed just at the point where, as Mike Ryan said:
“I knew we were in trouble.
“And trouble wasn’t long in coming!”
That trouble all started out at the exit — the exit event, that is.
Remember how Al Jackson and Mike Ryan predicted there’d have been an “exit event” later the same day as the impact? Here’s how they put it in their 1973 Nature article: 
“Because of its high velocity and because it loses only a small fraction of its energy in passing through the Earth, the black hole should very nearly follow a straight line through the Earth, entering at 30° to the horizon and leaving through the North Atlantic in the region 40°-50° N, 30°-40° W.”
Well, they did more than predict it; they went and bet the farm on it:
“This exit provides a check for the whole hypothesis. At the exit point there would be another air shock wave and an underwater shock wave and disturbance of the sea surface. Microbarograph records could be checked for an event similar to that caused by the entry shock displaced by the proper amount of time. Oceanographic and shipping records could be studied to see if any surface or underwater disturbances were observed.”
Now, don’t get me wrong: When a new theory makes empirically-testable predictions, that’s a good thing. And the crazier the theory, the more it’s got to have that kind of verification. It was this same sort of thing, for instance, that finally brought Newtonian physicists around to Einstein’s way of thinking, once a team led by Arthur Eddington team found that the precession of Mercury during the 1919 solar eclipse exacly matched the predictions of General Relativity.
Still and all, it was because of that “exit event” prediction that Mike and Al got to watch their brand-new hypothesis go down in flames not even a year after its publication.
As Mike recalls it:
“We had calculated that it would come out in the North Atlantic — if it had come out in New York City there was absolutely no possibility. And so, um, somebody had actually gone and looked at the microbarograph records for the time that it would have taken the shockwave — an exit shockwave — to get from the North Atlantic to London, and there was nothing there.”
And, most mortifying of all, it was a couple of naturalized fellow Texans that did the looking.
In August 1974, William H. Beasley and Brian A. Tinsley from the University of Texas at Dallas wrote in to Nature (the same journal that had published Mike and Al’s original Tunguska article) to say “We have examined copies of the English microbarograph records, but have been unable to find any sign of waves from the suggested exit explosion.” 
There was more, but what it came down to in the end, was:
“All the evidence favours the idea that the impact which caused the Tunguska catastrophe involved a body with characteristics like a cometary nucleus, rather than a black hole.”
That wasn’t all.
Even as he was writing up the original article, Mike says he’d been worried about what other detectable effects the hole might’ve had, both on impacting the earth, and while tunneling through it. In the end, he and Al went with some handwaving about how “the black hole would leave no crater or material residue” as it plunged into the earth at the impact site, and how once inside the earth “the rigidity of rock would allow no underground shock wave.” Or, as Mike tells it:
“And then we assumed that — we didn’t assume, I just wrote it down in one sentence to make sure that we didn’t have to calculate anything — that the earth was good and solid, so this thing, once it hit the earth, would stop producing a shockwave.”
In retrospect, Mike and Al probably should have run those calculations, just for kicks and giggles.
Because somebody else sure did. Somebody else name of Jack Burns, George Greenstein, and Ken Verosub.
And what they came up with drove the final nail in the coffin. Not that their 1976 paper in the Monthly Notes of the Royal Astronomical Society didn’t start out promisingly enough: 
“The apparent uniqueness of this [Tunguska] event requires that all possible explanations must be seriously considered and that no explanation can be discarded merely because it has a low probability of occurring.”
So far, so good. But it all went downhill from there.
Burns, Greenstein, and Verosub started out with the power of the impact — estimated by E. L . Krinov at somewhere between ten and forty megatons — and worked backwards.
Now, back in the early seventies, before the discovery of things like Hawking radiation and black monopoles, you’d’ve had to assume that all that energy had to come from gravitational effects alone. A submicroscopic-sized black hole is way too small and way too dense to generate anything in the way of atmospheric friction, you see. The only way it’s got of releasing energy into the atmosphere is by its gravity tugging directly on the air molecules. But that’d take a powerful gravitational field for sure. According to Burns, Greenberg, and Verosub, putting out that much gravity meant the Jackson-Ryan hole would’ve had to’ve weighed in somewhere between ten quadrillion and one quintillion tons. That’s a little on the high side versus Al and Mike’s own quadrillion-ton guesstimate, but what’s an order of magnitude or three among friends?
Then Burns and company took it one step further. They figured what would happen when that same mass, that same gravitational energy touched down on earth.
According to them —
“[T]he point of entry of the hole into the Earth should be marked by a patch of melted and resolidified rock of diameter one-half to four kilometres, overlain by fused soil of comparable extent.”
Wow! A disk of fused earth and melted rock maybe three miles wide and Lord knows how thick. Bet that’d have been hard to miss!
But Burns et al. weren’t done yet. They also went and calculated the effect of the hole’s gazillion-ton mass burrowing through thousands of miles of the earth’s crust and mantle. The answer?
Well, here, let’s let Mike Ryan tell it:
“I have here a list — as old Joe McCarthy said — of twenty-five articles. One of them that was critical, of course, took the idea that the earth is not all that solid, so it’s not just going to punch a hole through the earth, and the shockwave is going to continue. And the calculation that’s in one of the articles said that it was something on the order of all of the earthquakes in one year that the earth has ever felt, at this one time, as it’s going through the earth. I think they said something like an 8.5 earthquake for every kilometer that the thing moved through the earth.”
“Several thousand simultaneous earthquakes,” each of them as big or bigger than the most powerful earthquake ever recorded, was what the article said.
Getting a sinking feeling yet? There’s more …
All quotes from Al Jackson and Mike Ryan are courtesy of Albert A. Jackson, IV and Michael P. Ryan, Jr., as recorded at the Johnson Space Center Amateur Astronomical Society meeting and Singularity launch party, November 2004.
 Albert A. Jackson, IV and Michael P. Ryan, Jr., “Was the Tungus Event due to a Black Hole?” Nature, vol. 245, September 14, 1973, pp. 88-89.
 William H. Beasley and Brian A. Tinsley, “Tunguska Event was not caused by a black hole,” Nature, 250 (1974), pp. 555-556, https://www.nature.com/articles/250555a0/.
 Jack O. Burns, George Greenstein, and Kenneth L. Verosub, “The Tungus Event as a Small Black Hole,” Monthly Notices, Royal Astronomical Society, vol. 175 (1976), pp. 355-357.