For the most part Africa is surrounded by sea and so its boundaries are clear. Where it meets Asia the boundary is generally thought to be the man made Suez Canal and the Red Sea. This places the Sinai and part of Egypt in Asia. Africa is the second largest continent by area and by population, about 30.3 million km2 (11.7 million sq miles) and 1.6 billion people. Africa's average population is the youngest amongst all the continents, the median age in 2012 was 19.7, when the worldwide median age was 30.4. As an extremely young continent and with its enormous natural resources it has the potential to grow more than any other continent, and it is thought that in the next few years Africa will become a much bigger player on the world economic stage. The Mediterranean regions and areas to the North of the Sahara Desert were influenced by major empires such as Egypt, Rome and various ones that were Islamic. In Sub-Saharan Africa the area remained mainly influence free from outside Sates, and so several larger polities sprung up among them was the Kingdom of Nri, famed for its beautiful bronzes, which eventually became part of Nigeria. Slavery was always a profitable trade, about 18 million slaves were taken to Arabic countries, lasting from the 7th century to 20th century. The Atlantic slave trade, up to about 12 million, was from the 15th to the 19th Century. There was huge colonization by European nations from the end of the 19th century, and in some places this only finished about a century later. African countries have been held back by corruption and by poor infrastructure. There has been huge investment in the last 20 years especially from China. Israel is forming many trade and diplomatic relations with African countries but this continues to be stymied by the influence of Muslim countries. The continent is in both the Northern and Southern Hemispheres, it has a large population that live on the Equator, and consequently the climate varies. There are many beautiful regions to visit in Africa and it is also largely untapped as a tourist destination.