Suggested mechanism at around 18 minutes.

Earth was formed with the core being elemental Iron.

Slowly over time, water seeps to the core, and the Elemental iron oxidizes, making it physically larger. Eight times larger.

Fe + 3H2O ==> Fe2O3 + 3H2

Earth did not appreciably change in mass. (The orbit would have been affected).

But I think it is plausible that the iron core simply rusted and grew in volume as a result. This would have bound up a great deal of Earth's oxygen, and hence water.

The author suggests that the Earth began covered mostly in water and points to the existence of marine fossils found in the mountains as evidence.

Science! :)

By Jove I think he might be right!