In Genesis, the floodwaters rose fifteen cubits above the mountains during Noah’s Ark story.
Genesis 7:20 gives the figure after describing the rising waters: they covered the mountains to a depth of fifteen cubits. Because an ancient cubit was a body-based unit whose length varied, the passage does not provide one universally agreed modern measurement. Common estimates place a cubit around 44–52 centimetres, making fifteen cubits roughly 6.6–7.8 metres.
This number is distinct from the ark’s dimensions. Genesis gives the vessel’s length, width, and height separately, while the fifteen-cubit statement describes the floodwater’s depth above the mountains.
Readers also sometimes connect this verse with modern geological claims about a worldwide flood. The text itself is a narrative measurement; it does not identify the mountains by modern names or supply a scientific survey of water levels.