The distance from the surface of Earth to the center is 3,959 miles, which isn’t so very far. It has been calculated that if you sunk a well to the center and dropped a brick into it, it would take only forty-five minutes for it to hit the bottom (though at that point it would be weightless since all the Earth’s gravity would be above and around it rather than beneath it). Our own attempts to penetrate toward the middle have been modest indeed. One or two South African gold mines reach to a depth of two miles, but most mines on Earth go no more than about a quarter of a mile beneath the surface. If the planet were an apple, we wouldn’t yet have broken through the skin. Indeed, we haven’t even come close.

Until slightly under a century ago, what the best-informed scientific minds knew about Earth’s interior was not much more than what a coal miner knew-namely, that you could dig down through soil for a distance and then you’d hit rock and that was about it. Then in 1906, an Irish geologist named R. D. Oldham, while examining some seismograph readings from an earthquake in Guatemala, noticed that certain shock waves had penetrated to a point deep within the Earth and then bounced off at an angle, as if they had encountered some kind of barrier. From this he deduced that the Earth has a core. Three years later a Croatian seismologist named Andrija Mohorovii´c was studying graphs from an earthquake in Zagreb when he noticed a similar odd deflection, but at a shallower level. He had discovered the boundary between the crust and the layer immediately below, the mantle; this zone has been known ever since as the Mohorovii;c discontinuity, or Moho for short.

We were beginning to get a vague idea of the Earth’s layered interior-though it really was only vague. Not until 1936 did a Danish scientist named Inge Lehmann, studying seismographs of earthquakes in New Zealand, discover that there were two cores-an inner one that we now believe to be solid and an outer one (the one that Oldham had detected) that is thought to be liquid and the seat of magnetism.

At just about the time that Lehmann was refining our basic understanding of the Earth’s interior by studying the seismic waves of earthquakes, two geologists at Caltech in California were devising a way to make comparisons between one earthquake and the next. They were Charles Richter and Beno Gutenberg, though for reasons that have nothing to do with fairness the scale became known almost at once as Richter’s alone. (It has nothing to do with Richter either. A modest fellow, he never referred to the scale by his own name, but always called it “the Magnitude Scale.”)

The Richter scale has always been widely misunderstood by nonscientists, though perhaps a little less so now than in its early days when visitors to Richter’s office often asked to see his celebrated scale, thinking it was some kind of machine. The scale is of course more an idea than an object, an arbitrary measure of the Earth’s tremblings based on surface measurements. It rises exponentially, so that a 7.3 quake is fifty times more powerful than a 6.3 earthquake and 2,500 times more powerful than a 5.3 earthquake.

At least theoretically, there is no upper limit for an earthquake-nor, come to that, a lower limit. The scale is a simple measure of force, but says nothing about damage. A magnitude 7 quake happening deep in the mantle-say, four hundred miles down-might cause no surface damage at all, while a significantly smaller one happening just four miles under the surface could wreak widespread devastation. Much, too, depends on the nature of the subsoil, the quake’s duration, the frequency and severity of aftershocks, and the physical setting of the affected area. All this means that the most fearsome quakes are not necessarily the most forceful, though force obviously counts for a lot.

The largest earthquake since the scale’s invention was (depending on which source you credit) either one centered on Prince William Sound in Alaska in March 1964, which measured 9.2 on the Richter scale, or one in the Pacific Ocean off the coast of Chile in 1960, which was initially logged at 8.6 magnitude but later revised upward by some authorities (including the United States Geological Survey) to a truly grand-scale 9.5. As you will gather from this, measuring earthquakes is not always an exact science, particularly when interpreting readings from remote locations. At all events, both quakes were whopping. The 1960 quake not only caused widespread damage across coastal South America, but also set off a giant tsunami that rolled six thousand miles across the Pacific and slapped away much of downtown Hilo, Hawaii, destroying five hundred buildings and killing sixty people. Similar wave surges claimed yet more victims as far away as Japan and the Philippines.

For pure, focused, devastation, however, probably the most intense earthquake in recorded history was one that struck-and essentially shook to pieces-Lisbon, Portugal, on All Saints Day (November 1), 1755. Just before ten in the morning, the city was hit by a sudden sideways lurch now estimated at magnitude 9.0 and shaken ferociously for seven full minutes. The convulsive force was so great that the water rushed out of the city’s harbor and returned in a wave fifty feet high, adding to the destruction. When at last the motion ceased, survivors enjoyed just three minutes of calm before a second shock came, only slightly less severe than the first. A third and final shock followed two hours later. At the end of it all, sixty thousand people were dead and virtually every building for miles reduced to rubble. The San Francisco earthquake of 1906, for comparison, measured an estimated 7.8 on the Richter scale and lasted less than thirty seconds.

Earthquakes are fairly common. Every day on average somewhere in the world there are two of magnitude 2.0 or greater-that’s enough to give anyone nearby a pretty good jolt. Although they tend to cluster in certain places-notably around the rim of the Pacific-they can occur almost anywhere. In the United States, only Florida, eastern Texas, and the upper Midwest seem-so far-to be almost entirely immune. New England has had two quakes of magnitude 6.0 or greater in the last two hundred years. In April 2002, the region experienced a 5.1 magnitude shaking in a quake near Lake Champlain on the New York-Vermont border, causing extensive local damage and (I can attest) knocking pictures from walls and children from beds as far away as New Hampshire.

The most common types of earthquakes are those where two plates meet, as in California along the San Andreas Fault. As the plates push against each other, pressures build up until one or the other gives way. In general, the longer the interval between quakes, the greater the pent-up pressure and thus the greater the scope for a really big jolt. This is a particular worry for Tokyo, which Bill McGuire, a hazards specialist at University College London, describes as “the city waiting to die” (not a motto you will find on many tourism leaflets). Tokyo stands on the boundary of three tectonic plates in a country already well known for its seismic instability. In 1995, as you will remember, the city of Kobe, three hundred miles to the west, was struck by a magnitude 7.2 quake, which killed 6,394 people. The damage was estimated at $99 billion. But that was as nothing-well, as comparatively little-compared with what may await Tokyo.

Tokyo has already suffered one of the most devastating earthquakes in modern times. On September 1, 1923, just before noon, the city was hit by what is known as the Great Kanto quake-an event more than ten times more powerful than Kobe’s earthquake. Two hundred thousand people were killed. Since that time, Tokyo has been eerily quiet, so the strain beneath the surface has been building for eighty years. Eventually it is bound to snap. In 1923, Tokyo had a population of about three million. Today it is approaching thirty million. Nobody cares to guess how many people might die, but the potential economic cost has been put as high as $7 trillion.