In real estate, pricing is relative. Some cities are outrageously expensive and become even more so, while others are bargains.

It would surprise no one that the most expensive real estate markets in the world are the most densely populated. There are simply few, if any, places to build, so prices stay high and keep rising.

Cities hemmed in by mountains or water have this problem, while a metropolis bordered by generous amounts of open space do not. Of course, that doesn’t mean that prices in the most-expensive markets are fair. They are often headed for a fall.

In a recent survey, the Swiss-based bank UBS compiled a list of the most expensive cities in their Real Estate Bubble Index. You don’t need a drumroll to know the most bubble-prone markets: Hong Kong, London, Paris, Singapore, Toyko, New York and Amsterdam top the list. Most of these cities would qualify as “hemmed in” by geographic obstacles. There’s simply no more room to build.