Subduction processes on the seabed where continental plates go under another, cause very water-rich rocks to be drawn down into the Earth's interior.
The water that goes down in the mantle does not reappear until long time afterwards, for example in the case of volcanic activity. The new study is done using mathematical calculations and modelling. Read more about this research in the articles below.
Read articles:
The oceans are draining below Earth’s crust, NewScientist, paper magazine, 1.06.2019
The oceans are very slowly draining into the rock below Earth's crustv, New Scientist, Online, 24.05.2019
The Oceans are slowly leaking into the Earth. Futurism.com, 24.05.2019