Where every figure comes from

Every number on the site is read from a named source, and the page that uses it lists that source at the bottom. We prefer the organisation or paper that made the measurement: the International Commission on Stratigraphy for dates, the Preliminary Reference Earth Model for depths and pressures, the US Geological Survey, the British Geological Survey and peer-reviewed papers for the rest. Where we use an encyclopedia, we say so, and point to what it cites.

Where a figure is our own sum, such as a distance between cities or how much wider the Atlantic is since 1492, the page says so. Where scientists disagree, such as the temperature at the centre of the Earth, we give the range. Dates in deep time are rounded the way the source rounds them.

Every drawing is made in code for this site. They show ideas, not photographs, and each caption says what is simplified. The colours of geological time are the official ones; the colours of the Earth's layers are chosen for the site.

If you think something is wrong, email [email protected] with the page and, if you can, a source. We fix errors and note the date.