London (CNN) – In December 1991, I was a 27-year old journalist covering the creation of a European treaty in Maastricht. Negotiations took some weeks, but exactly twenty years ago on Tuesday the city, sandwiched between Belgium and Germany, witnessed the signing of "The Treaty on European Union." The European Union had been created.

I recall forgetting my pass (yes, that one pictured above) and having to get a taxi back to our hotel in Germany to get it. And I remember the story being a bit confusing. I had moved from the U.S. to Europe only a year earlier and was trying to figure how things differed across the Atlantic.

I wondered if the so-called Maastricht Treaty really was creating a "United States of Europe" as some called it? And why did Britain say it had an "opt in" to the treaty, when everyone else called it an "opt-out"? Would the French, Italians and Dutch really be happy to scrap their centuries-old currencies for a new one, yet unnamed?
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Hong Kong (CNN) – Hong Kong is one of the most densely populated places on the planet.

In a place like this, one of the greatest challenges is getting people from A to B.

The city's main subway carries 4 million passengers a day. This key piece of infrastructure is being expanded.

CNN's Richard Quest goes beneath the city streets to find out how it will be done.