Border security

World of walls: How 65 countries have erected fences on their borders – four times as many as when the Berlin Wall was toppled – as governments try to hold back the tide of migrants

From Israel's separation barrier (or 'apartheid wall' as it is known by the Palestinians), to the 2,500-mile barbed-wire fence India is building around Bangladesh, to the enormous sand 'berm' that separates Morocco from rebel-held parts of the Western Sahara – walls and fences are ever-more popular with politicians wanting to look tough on migration and security.

US presidential hopeful Donald Trump has made plans for a wall along the border with Mexico – to keep out what he called 'criminals, drug dealers, rapists' – central to his inflammatory campaign.

Yet experts say there is little proof of their effectiveness in stopping people crossing borders. 

In July, Hungary's right-wing government began building a four-metre-high (13 feet) fence along its border with Serbia to stanch the flow of refugees from Syria, Iraq and Afghanistan.