The ‘open sea’ is taken to be water whose depth is greater than 150-200m, which in turn marks the end of the continental shelf. The continental shelf is the tip of the continents which has been flooded by shallow seas and gulfs ever since the end of the last Ice Age. This means that for you to have left completely the continent of Europe, you must fly out over sea which is at least 150-200m deep!

The zone within which enough light penetrates for photosynthesis to occur is known as the euphotic zone, and generally extends to a maximum of 50m. The depth of this zone depends on the transparency of the water. Some small quantities of light still penetrate up to 600m, which is then followed by a number of different dark zones, the deepest of which is known as the Hadal Zone (Hades – Greek God of the Underworld).