There is compelling evidence that the poles have shifted in the past, but this idea is dismissed as impossible by the scientific community on the assumption that the stabilizing effect of the equatorial bulge is so great that no conceivable force could make the Earth shifting on its axis, except for the collision with a planet-size body. In theory, however, a wide shift of the poles could be obtained simply by reshaping the equatorial bulge, a ring of matter that from about 15km at the equator decreases down to zero at the poles. At least 20% of this matter is made by water, which covers 2/3d of the whole Earth. A well-known physical law assures that free liquid surfaces create instability, thus Earth is an inherently unstable planet. Every displacement of water provokes a wobbling of the axis of rotation. An ocean wide tide or tsunami of hundreds of meters would displace the axis of some degrees, therefore the polar icecaps would rotate off-center developing a toppling torque. The shift would increasingly grow to the point of provoking the sudden rebound of the Earth’s mantle and in the end a reshaping of the equatorial bulge around a different axis of rotation. We can imagine more than one reason that in theory could provoke a tide of the required magnitude, but the most probable culprit should be the impact of a large asteroid. The analysis of the behaviour of a gyroscope subject to a disturbing torque provides a clear explanation of why and how the impulsive torque produced by the impact of an asteroid could trigger a process which in the end results in a shift of the poles.