British Airways, Cabin Failed to Pressurize

A British Airways Boeing 747-400, registration G-CIVU performing flight BA-242 from Mexico City (Mexico) to London Heathrow,EN (UK), was climbing out of Mexico City when the crew stopped the climb at 12000 feet reporting a technical issue, adding pressurization issue, and entered a hold to work the checklists, then decided to return to Mexico City, dumped fuel and landed safely back about 2:10 hours after departure. The aircraft remained on the ground for about 2 hours, then departed again and reached London with a delay of 4 hours.

Source: http://www.avherald.com/h?article=4abcf642&opt=0