According to the website:

3. Dredging in the Fehmarnbelt and establishment of new land areas

An 18 km long trench from Denmark to Germany will be dredged to accommodate the tunnel elements. The tunnel elements will be immersed into the trench, joined together and covered with gravel, sand and stone. The surplus material from the dredging will be used to construct new coastal areas near Rødbyhavn and to a limited extent on Fehmarn.

Work on dredging the tunnel trench began in 2021 and is expected to continue until 2024.