Answer: Europe and Asia
There is no ocean or channel, nor an edge of tectonic plate separating Europe from Asia, for that reason many geographers maintain that Europe and Asia share many common geographical characteristics and that, in reality, they are part of a single continent, which they named Eurasia.
In fact, the borders between these two continents are due more to sociopolitical than geographical reasons. Well, geographically Europe is considered a great peninsula of Asia.
However, the current definition of the boundary between Europe and Asia begins in the Aegean Sea following the Dardanelles-Sea of Marmara-Bosphorus, the Black Sea, following the Great Caucasus basin, the northwestern part of the Caspian Sea and along the Ural mountains.