The occurrence of natural radioactive carbon in the atmosphere allows archaeologists the ability to date organic materials as old as 50,000 years. Carbon-14 is continuously produced in the atmosphere and decays with a half-life of 5,730-year (+/- 40 years). Unlike most isotopic dating methods, the carbon-14 dating technique relies on the progressive decay or disappearance of the radioactive parent with time. This is now a common method for estimating the age of a carbonaceous archaeological artifacts. The radioactivity of an artifact's carbon-14 content determines how long ago the specimen was separated from equilibrium with the atmosphere-plant-animal cycle. The method is based on the principle that all plants and animals, while they are alive, take in small amounts of carbon-14 and when they die, the intake ends. By measuring the loss rate of the carbon 14, the age of the object can be established. Measurement of the carbon-14 activity in a cypress beam in the tomb of the Egyptian Pharaoh Snefru, for example, established the date of the tomb as c 2600 BC.