Ever since the breakup of the supercontinent Pangaea, sea water has been flowing deep into the planet, causing sea levels to fall over millions of years
Source:
https://www.newscientist.com/article/2204528-the-oceans-are-very-slowly-draining-into-the-rock-below-earths-crust/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home