Ontology evolution is an intrinsic phenomenon of any knowledge-intensive system, which can be addressed either implicitly or explicitly. This paper describes an approach to capture and visualize implicit data-driven ontology evolution using ontologies semi-automatically generated by extending small seed ontologies. This process captures ontology changes reflected in large document collections. Visualizing of these changes helps characterize the evolution process, and distinguish core, extended and peripheral relations between concepts. Finally, the paper presents an example of ontology evolution by monitoring and analyzing online media coverage on "energy sources" over a period of ten months.