The exploration of even the outermost fringes of our Solar System has long crossed over from the realm of fantasy to that of reality. But the human quest to explore our planet's interior — magically expressed in Jules Verne's epic A Journey to the Centre of the Earth — lags far behind. Unlike the leap into space, gravity defying as it may be, a voyage into the planet's bowels faces the sheer resistance of drilling through kilometre after kilometre of dense rock.

Human innovation, though, is only fuelled by such challenges. Indeed, penetrating the oceanic crust and traversing the crust–mantle boundary (the Mohorovičić discontinuity, or Moho) is the principal objective of the ambitious Mission Moho initiative, discussed at the September workshop of the Integrated Ocean Drilling Programme (IODP) in Bremen, Germany.

The first attempt at accomplishing this daunting task — made in the 1960s — was unsuccessful, and the hole reached a depth beneath the sea floor of less than 200 m. But it showed that drilling in the deep ocean was possible, and inspired subsequent missions that have managed to drill deeper. The data and insights gained from such projects have radically altered our understanding of the evolution of the oceanic crust and uppermost mantle.

However, so far no deep-sea drill has managed to reach beyond 1.5 km beneath the sea floor, which is only about one fourth of the typical distance from the sea floor to the Moho. As a result, much of our knowledge about the deeper parts of the oceanic crust and the uppermost mantle comes from observations of slices of oceanic plates that have been shoved on to continents by plate tectonic processes.

But geological exposure is incomplete as a rule, and remnants of the mantle that have ended up on the continents are not pristine. More importantly, the diverse processes that went into the construction and modification of oceanic crust and uppermost mantle cannot be directly observed in such outcrops, but can only be inferred. Drilling through the oceanic lithosphere is expected to fill in the gaps in our understanding of processes ranging from mantle melting to hydrothermal alteration.

Mission Moho is still in its infancy, and its success will depend on technological feasibility as well as budgetary considerations. The former may not be an obstacle for too long. Recently developed technology is likely to allow drilling through sea floor that lies at depths of over 4 km beneath the sea surface: this will enable the relatively thin oceanic crust along a mid-ocean ridge to be targeted. In contrast, current technologies can negotiate a water depth of only about 2.5 km.

Drilling into the mantle is not expected to be cheap by any means. But in all likelihood, Mission Moho will only cost a fraction of what is currently spent on space exploration: compare, for example, the current IODP annual budget of about $200 million (Nature 461, 578–579; 2009) with that of NASA (over $15 billion for 2009). Peeking into the Earth's interior is closer to home. The voyage is well worth undertaking.