Mathematica is a large and complex software system. Although strongly not supported for production purposes, it is sometimes instructive to "spelunk" in the system, looking at low-level organization and obscure features.
Names — complete lists of symbol names matching a pattern (e.g. )
Contexts — give complete lists of contexts for symbol names
SystemOptions — low-level system options
Show Expression (Shift+Ctrl+E) — show the symbolic structure of notebook contents
Option Inspector — inspect lists of front end options
Trace — trace every step in the evaluation of an expression
Stack — show the state of the complete evaluation stack