“The heat and the altitude worried everybody. The 1970 World Cup in Mexico would not be a normal one. So the Bulgarian authorities sent their squad south of Sofia to get used to playing several thousand feet above sea level. Which seemed a great idea until somebody noticed that the temperature in the Pirin mountains was not in the mid-20s as it is in Mexico but somewhere near freezing. How then could they replicate the effect of playing in intense heat? By restricting water intake so that the players got used to performing while dehydrated. The plan was not a great success. …” – Jonathan Wilson
Guardian