They’ve made some remarkable advances in the study of the core and mantle, in part due to advances and greater numbers of seismometers. It’s a very difficult thing to study.
Quite true, and I marvel at what scientists have accomplished so far. For example, I read recently that a Japanese team had measured the change in the level of neutrinos detected when a local nuclear plant was briefly shut down. The residual was seen as a measure of nuclear decay processes at work deep in the earth.