But the tour guides often talk about strange geophysical influences that baffle scientists. Is that hokum, or might genuine Earth effects having to do with magnetism and gravity be active here too?
I don't accept that any magnetic or gravity anomalies are present at these sites. For instance, inside one of these trick houses a compass might behave oddly if you can't tell when you're holding it level.
That said, compasses do vary slightly from place to place and time to time. Sometimes they respond to nearby bodies of iron ore, or places where lightning strikes have magnetized rock outcrops. Sometimes geomagnetic storms, like those that cause auroras, make compass needles fluctuate. There are also the dimly understood electrical currents, called telluric currents, that flow deep underground where rocks can conduct electricity.
But compasses also are affected by human causes, like electric power lines. (Remember your middle-school physics class: moving magnets generate electricity in a wire coil, and vice versa—that's how electric motors work.) Electrically powered trains send out powerful magnetic noise. And natural electromagnetic forces can interact in odd ways with large structures like pipelines or bridges.
But remember that "mystery spots" are, at root, no different from magicians' acts or amusement parks. It would be trivial to bury electric wires around a "mystery house" and disrupt compasses at will. If you suspect trickery—and in human affairs you always should—it would take a careful and skeptical study to investigate this subject.
As for gravity, there are absolutely no gravity effects to be found at these places. The force of gravity does vary, very slightly, from place to place. That reflects the density of the rocks in the area and, on a larger scale, the balance of tectonic forces in the region. If tectonic forces are holding the area higher (or lower) than it would otherwise be, the measured gravity there is very slightly smaller (or greater).
True gravity anomalies are very subtle things, detected only with exquisitely sensitive equipment and extensive analysis. They're also quite large in scale, nothing that would be noticeable at a particular mystery spot.
Mystery Spots Are Still Fun
Of course certain places can affect the mind, sometimes powerfully. Over the years, local people may turn these into sacred sites. Stories arise about them and persist. I argue that this is a result of human nature.
Some mystery spots from ancient history acquired great power indeed. Probably the most famous example is the Oracle of Delphi, in ancient Greece, a dramatic cleft in a high hillside where trained women would fall into trances and utter prophecy inspired by the god Apollo. Recent geologic discoveries there point to a plausible explanation that intoxicating gases came up from the ground there.
Today is a scientific age, in which tales of gods and occult influences don't have all the power they used to. Perhaps the appeal of "mystery spots," persistent and popular as they remain, is also just a sign of deep human nature. That's OK, but science gives them no support.