Presumably it won't melt, because the melting point of tungsten is higher than that of the layers through which it passes. It will be best if its instruments can function at ambient temperature and pressure, since it will be difficult to maintain an instrument compartment at surface temperature and pressure. Multiple sources will be wanted. Presumably a nuclear reactor powered heat engine, but it must have a T 1 greater than ambient, and a T 2 at ambient or somewhat higher, so it will be a different kind of nuclear reactor and a different kind of heat engine than is used on the surface. This seems chancy.
Information is coded in neutrinos produced by an accelerator.
This would require quite a large neutrino detector, but who says this project would be cheap. If none of these worked, maybe seismic waves would bounce off it detectably, at least giving its current location. I hope to provide some numbers in a future web page, but it would be better if they were provided by someone with better qualifications than mine. I sometimes make changes suggested in them. The number of hits on this page since 1999 September 10.