The earth was originally created for the angels, perhaps as long ago as science calculates its age.

They 'kept not' (maintained) their estate, instead rebelled with Lucifer.

They were cast (back) to earth. This caused massive destruction, upheaval, etc.

They were consigned to inhabit creatures especially made by God to reflect their wicked nature. This continued for perhaps millions of years until God began his plan of restoration.

During this time the earth continued to suffer flooding, earthquakes, upheavals, etc. from the destructive forces set in motion by the casting down. Generations of these creatures died in the turmoil and were buried in the sediments.

The first stage of the restoration was to destroy that ancient world, by a final worldwide flood.

Genesis 1:2 describes the earth in that flooded condition.