About 12 miles (20 km) from the shores of Lake Titicaca lie the ruins of what was once the site of the greatest city in South America. Thought by some to date back to 15,000 BC when it might have been on the shore of the lake, it is now considered by radio carbon dating to have begun around 1500 BC and flourished from around 300 to 1000 AD when it probably came to an end due to climate change - a worsening climate meant the area was no longer able to support the industrious agricultural population. One of the most fascinating aspects of the city is the famous Sun Gate, carved with icons said to relate to a calendar. To the side of the gate is a wall which contained 11 pillars and these are the actual calendar, marking the time of the year according to the position where the sun set every night and dividing the year into 20 fortnights of 18 days or 10 months of 36 days which also meshed every three years with a lunar calendar of 40 sidereal months making it one of the most sophisticated yet less known calendars in the world. This magnificient city was completely destroyed by the Spanish Conquistadores of the sixteenth century and the little that remained was further smashed up and used as ballast for the construction of the nearby railway. "Tiwanaku: a City Lost in Time" is an attempt to gather the remaining photographic evidence of the city from the time when it was first documented by Europeans and piece together the actual workings of the calendar and the system of cubits that was used in the construction of the monuments such as the Akapana pyramid, built so that one side marked a year of 360 days and the other the actual year of 3651/4 days, whilst some monuments were designed in "Egyptian" cubits, others in "Sumerian" cubits and finally we discover the length of the "Tiwanaku" cubit itself.