Scientists will set out this week to drill a hole into the Indian Ocean floor to try to get below the Earth's crust for the first time.
They want to sample rock from the planet's mantle - its deep interior.
In the process, the researchers hope to check their assumptions about the materials from which the crust itself is made.
It will probably take several years to drop the full 5 to 5.5km, says co-team leader, Prof Chris MacLeod.
This is in addition to the 700m of water between the drilling ship, the Joides Resolution (JR), and the seabed.
There have been several attempts to drill into the mantle, but none has yet succeeded.
This latest effort may fare better, however, because faulting and erosion have already thinned the crust at the targeted drill site, known as Atlantis Bank on the South West Indian Ridge of the Indian Ocean.
The project, which is running under the auspices of the International Ocean Discovery Program (IODP), would give scientists access ultimately to fresh, unaltered peridotite - the rock, rich in olivine minerals, that, because of the size of the mantle, makes up the bulk material of the planet's interior.
This is a worthwhile goal in itself. But as they sink deeper and deeper, the researchers want also to test current models for how the crust is constructed and constituted.
In particular, Prof MacLeod is keen to probe the so-called Moho boundary.
This is the famous "discontinuity" where seismic waves from earthquakes abruptly change speed.
The textbook explanation is that the Moho draws the line between the crust and the mantle: a demarcation between familiar igneous surface rocks - such as granites, basalts and gabbros - and those of the interior peridotites.
Given that the seismic signature of this material is essentially the same as crustal igneous rocks, there is no way of telling - other than to drill and sample everything between the seabed and the top few hundred metres of unadulterated mantle.
If proven correct, the more sophisticated Moho description would have a number of far-reaching consequences for our understanding of how the planet is put together.
For a start, it would mean the igneous ocean crust is far more variable in thickness and in structure than previously recognized.