For the first time, rocks have been extracted from California's San Andreas Fault by scientists drilling some three kilometres below the surface.
They say this new material may answer a number of long-standing questions about the fault's composition and properties.
Previous work has had to estimate the nature of the rock at this depth.
Now the scientists from the San Andreas Fault Observatory at Depth (Safod) project are relishing the prospect of studying the real thing.
Professor Mark Zoback from Stanford University said: "Now we can hold the San Andreas Fault in our hands. We know what it's made of. We can study how it works."
The drill site is near the small town of Parkfield, where the fault is very active, moving regularly, but only creating relatively small earthquakes.
One area of particular interest centres on why, when most of the fault jolts in violent earthquakes, the area near Parkfield shifts in a series of small movements, a process known as creep.
The next phase of the experiment is to install an array of instruments in the long borehole that runs through the fault.
With these sensors, scientists hope to observe the earthquake generation process.
The Safod project is a collaboration between EarthScope, a National Science Foundation-funded programme, the US Geological Survey and the US space agency (Nasa).
Project drilled directly into San Andreas quake zone
It will measure changes in rock before, during, after tremors
Data will be compared with surface measurements
Steve Hickman, a geophysicist with the USGS said: "To an earthquake scientist, these cores are like the Apollo Moon rocks.
"Scientists from around the world are anxious to get their hands on them in the hope that they can help solve the mystery of how this major, active plate boundary works."
The 1,300km-long San Andreas "crack" that runs right through central California has been the cause of many big earthquakes, including the famous 1906 "Big One" which flattened San Francisco.
The fault traces the boundary between the Pacific and North American tectonic plates; and has become a major focus of earthquake research.
This past week, the international scientific community has begun an even grander drilling project off the coast of Japan.
The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is putting a bore hole into an area of ocean floor subduction - a different type of plate boundary.
It will retrieve rocks that are perhaps twice the depth of Safod.
The key prize for all such projects is to try to establish if any precursor signals exist that could be used to predict earthquakes reliably and accurately.