Big Ben's* widely held reputation as the world's most famous clock could be under jeopardy from this behemoth, which today begins ticking in the Saudi city of Mecca. At 601m tall, the Abraj Al-Bait tower will be second only to the Burj Dubai, and almost twice the height of London's under-construction Shard London Bridge. The clock alone has a diameter of 40 metres, five times the size of Big Ben's* faces. Bullish locals have provided a provocative news hook by suggesting that their new landmark could topple GMT as the world time standard, ushering in Mecca Mean Time. The $3 billion luxury hotel complex will greet millions of Muslims on the annual Hajj, a pilgrimage of humility and spiritual fulfilment.