In an effort to accurately map and predict earthquakes and tectonic plate movements, scientists wish to insert sensors through the earth's crust and into the upper mantle.
Through some Excel wizardy, they manage to get the funding to extend the Kola Superdeep borehole which currently extends to a little over 40,000ft (over 12km) deep. The Kola borehole was abandoned for budget reasons back in 2008 and the hole covered.
Some WD-40 and a bit of elbow grease should be enough to re-open it again.
Plot dictates the boreholes should be located at points of greatest stability (on land, away from tectonic plate boundaries). While it's possible to use volcanoes, there's reasoning that excessive magma movement would make accurate measurements next to impossible).
Should the Kola extension be successful, further boreholes will be drilled in other locations around the Earth and instruments inserted.
Assuming that "modern day" materials are able to withstand the heat of deep drilling, what difficulties would the drilling team face? By "modern day", I mean a near future where heat-resistant tools and sensors can be manufactured.