Q.  Is the Earth expanding?

A.  Yes, amazingly so. Over the last 200 million years, it has doubled in radius.

Q.  How do we know this is so?

A.  Back in the 1960s, a major sea-floor drilling program identified the age of the seafloor under all major oceans. This showed that all the current oceans had developed in the last 200 million years, and gave the dates when particular parts were created.

Q.  How does this prove expansion?

A.  It's a matter of topology, the only way the new areas could fit was on a regularly expanding globe. But there were also proofs from other considerations.

Q.   So, is the mass of the Earth increasing?

A.  No, expansion is a result of changes in the Earth's core, only recently explained (See XT807 - The Concore Model of planet and star interiors for details.

Q.  Where is there more detail on this?

A.  See Fixed-Earth and Expanding-Earth Theories -- Time for a Paradigm Shift?.