The usual problems of measurement and its meaning are complicated and magnified when the object of study is in principle and in fact inaccessible. When a phenomenon occurs in a place where our instruments cannot reach, what can the relation between the instrument, its reading, and the phenomenon be? This essay asks how researchers have addressed questions about inaccessible processes of Earth's magnetic field on the surface, at the edge of space and under its surface. This case takes us beyond (...) simple contrasts of inductive and deductive methods, or field sciences versus basic sciences, to a messier, more nuanced view of inherently complex sciences. The article focuses on investigations of Earth's magnetism and electricity, but its perspectives are applicable to many areas of recent science. (shrink)