Crust   The Earth’s outer shell. It is made of solid rock that is between 7-70 kilometres thick.   GDP   Gross Domestic Product, the total value of all the goods and services produced by a country in a year. It is often measured 'per capita' or per person, to make an average.   Human Development Index (HDI)   A more accurate way of measuring development that combines several different indicators. Learn how to calculate the HDI from http://hdr.undp.org/statistics/indices/hdi_calculator.cfm.   Infant mortality   The number of babies out of every thousand born that die before the age of one.   Mantle   The layer of the Earth’s interior that is made of mostly solid rock. The Mantle extends from the bottom of the Earth’s crust down to 2,900 kilometres deep inside the Earth.   Militia   Groups trained like soldiers but are not part of an official army.   Opencast   Mining minerals from the surface of the Earth.   Population density   The number of people per square kilometre.   Rain Shadow   The sheltered (leeward) side of an upland area. Here, the air is descending which means that it is warming up and less likely to produce rain.