Project Mohole Redux...

Geologist want to drill a hole all the way through the Earth's crust and take samples of the molten mantle.  This is far more difficult than most people realize, as this analogy hints at:

It will be the equivalent of dangling a steel string the width of a human hair in the deep end of a swimming pool and inserting it into a thimble 1/10 mm wide on the bottom, and then drilling a few meters into the foundations.

A tenth of a millimeter is roughly 1/250th of an inch. The drill “bits” used to drill through the very hard rock above the mantle only last for about 50 hours of drilling.  After each bit is worn out, the entire “string” of steel rod, miles long, has to be extracted, the working end brought to the surface, the bit replaced, and the entire thing re-inserted in the 30cm (roughly one foot) diameter hole in the sea floor.  To do this just once is an amazing feat.  To get to the mantle, they'll have to do it many dozens of times...