We almost certainly did at one point, but life and tides can fill-in the gaps. We also truly do not know what terrain is underneath Antarctica, Greenland, etc.

What? Yes, we do.

https://science.nasa.gov/earth/earth-observatory/uncovering-...

https://sealevel.nasa.gov/resources/75/the-bedrock-beneath-a...

https://commons.wikimedia.org/wiki/File:AA_bedrock_bedmap2.4...