It could rupture at any moment, but drilling a 1.3 kilometre-deep hole in the South Island's Alpine Fault will help improve New Zealand's ability to forecast earthquakes.
The Alpine Fault is the boundary between the Pacific and Australian tectonic plates that scientists say moves about 27 metres every 1000 years.
Visible from space, it extends for about 650km from south of Fiordland along the spine of the Southern Alps and into Marlborough.
There is evidence the fault has ruptured 24 times in the past 8000 years, or every 330 years on average.
The last time it happened was in 1717.
Scientists say the ruptures produce magnitude-eight earthquakes that cause tremors throughout much of the South Island.
There is a 28 per cent chance it will happen in the next 50 years, which is high by global standards.
That hasn't fazed plans by a Kiwi-led team of international researchers to intersect the fault in October near Whataroa, north of Franz Josef on the west coast.
Once the borehole is complete, they will then lower a camera and other scanning equipment into the 10 centimetre-wide hole to measure its shape, geometry and rock structures.
They hope studying the inner workings of a major plate boundary fault will shed more light on the relationship between fluid pressure, the internal structure of the fault zone and the mechanics of earthquakes.
Scientists believe the large difference in fluid pressures on either side of the fault zone could play a role in how quakes begin.
The Deep Fault Drilling Project is led by a trio of researchers from GNS Science, Wellington's Victoria University, and the University of Otago.
Scientists and institutions from Canada, France, Germany, Japan, Britain and the US are also contributing to the study.
Victoria University's John Townend says the project will help scientists develop better models of shaking hazards which can be used by engineers and planners to forecast how the Alpine Fault will affect New Zealand.
"Ultimately we hope this investigation and ongoing monitoring of conditions within the fault zone will lead to a better understanding of how faults slip and generate seismic waves during large earthquakes, and what specifically is likely to happen in an Alpine Fault earthquake," Dr Townend said.
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