With the aid of a ring laser, scientists can record the tiniest variations in the Earth's spin...

...and detect the drift of whole continents

In a second world war bunker in the New Zealand countryside, 30m below the ground, tiny variations in the Earth's rotation are being measured.

The steady spin of our planet makes it a very good clock - but it is not perfect. The Earth's rotation axis twists and wobbles because it is not a perfect sphere, while the motion of its atmosphere reacts to the spin. One day can be a few thousandths of a second longer or shorter than the next.

Improving our knowledge of the Earth's twists and turns to a level of parts per billion is an important scientific goal, not least for precise satellite-based navigation.

Through radio astronomy and satellite laser ranging, we can see local motions near geological faults - even a modest earthquake causes the planet to wobble like a jelly relative to the stars. After only a few months, we can even detect continental drift.

But it always helps to combine results from different technologies. One way to measure the absolute rotation of an environment is to place a ring laser in it.

In simple lasers, the light beam bounces along a line between two mirrors. Ring lasers have mirrors arranged in a triangle or square, producing beams of light that lase both clockwise and anti-clockwise. If the laser is rotated, the beam that lases in the same direction as the spin becomes slightly redder and the counter-rotating beam slightly bluer. As a stand-alone sensor, a ring laser can give very useful information on variations in the earth's rotation.

Secret commercial and military developments in the 1970s and 1980s made ring lasers ever more sensitive to rotation, and they are now standard equipment in military and civilian aviation. But in true New Zealand, pacifist style, the Canterbury ring-laser programme - based at the University of Canterbury, Christchurch - is utilising these advances for pure science. In collaboration with the Technical University of Munich and the German Federal Institute for Cartography and Geodesy, we are building a suite of the world's most sensitive ring lasers in our underground laboratory at Cashmere, near Christchurch.

In the early 1990s, we built C-I, a ring with an area of 0.752m. It was unsophisticated, but we showed its scientific potential. Our German collaborators then built a second 12m ring laser called C-II, which could sense rotational motion down to one millionth of the Earth's rate of rotation.

C-II has become the prototype for the much larger G, which, with an area of 162m, is designed to reach one part in a billion of Earth rotation. Proof of principle has come from a, 122m pilot ring, dubbed G0, that can reach 10 parts per million precision.

C-II and G0 have already recorded previously unmeasured effects, such as the rotation that accompanies the waves caused by earthquakes. G is now under construction in Bavaria, at a cost of about Pounds 6 million.

Bob Dunn, of Hendrix College in the US, recently showed that a ring laser with a perimeter of 40m can operate as a gyroscope. Working with him, our enlarged New Zealand-German-US collaboration is building a simple ring laser at Cashmere, dubbed Ultra-G, with an area of 3702m - half the area of our cavern. Although less stable than G, Ultra-G is expected to be more sensitive and an excellent detector for seismic rotations.

Maybe Ultra-G will be stable enough to pick up some parts-per-billion effects in the Earth's rotation. Who knows, we may even find a surprise as we push open this new window for science.

Geoff Stedman is professor of physics at University of Canterbury, Christchurch, New Zealand.