The Mexicas built ancient Mexico City over an existing lake. They designed an ingeniously practical and creative system, a clear demonstration of their highly developed technological and agricultural skills. Floating armatures of wood-logs were constructed on the lake and covered with layers of mud that accumulated to provide arable land on which to grow food. Both the ancient Mexica capital city and the Spanish historic centre were constructed over the lake. Today, Mexico City is one of the most densely populated cities in the world. Water is in short supply and increasing amounts of groundwater are being extracted to meet the city’s needs, resulting in significant land subsidence problems. Monitoring the historic centre to understand how it is moving is now essential for its preservation!

The German Aerospace Centre (DLR), a UNESCO space partner of the ‘Open Initiative on the use of space technologies to support the World Heritage Convention’, has been assisting UNESCO with this issue. DLR used its TerraSAR-X radar satellite, imaging the city from space from September 2009 to January 2010 and tracking changes, as shown here.

The areas of the Mexican capital where TerraSAR-X has recorded the greatest terrain movements are shown in dark red. The green areas are those in which no changes were detected via the superposition of the radar images between 20 September 2009 and 30 January 2010. The radar-satellite images show that even within this short time period, the ground moved as much as 10 centimetres in some places as a result of the water extraction.The historic centre of Mexico City became a World Heritage site in 1987 and terrain subsidence is a major concern. Two of its most famous landmarks, the Bellas Artes Opera House and the cathedral, are sinking. DLR and UNESCO are assisting the Mexican conservation authorities by monitoring terrain movements in the area: space technologies assisting World Heritage sites.

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