On the main square of the old centre of the Sicilian city of Syracuse stands the Duomo di Siracusa. On the outside, it is a beautiful 18th century High Sicilian Baroque church.

On the inside, it is a Greek temple.

The columns that still support the roof of the church date to the 5th century BC. The building was originally a temple to Athena; it is mentioned in the writings of Plato (who lived in Syracuse for a while) and Cicero.

One of the more interesting examples of this, encompassing both my hobby and day job, is the case of the HMS Revenge and Royal Sovereign. Construction of the ships started in late 1913 and early 1914, and they were finished in 1916. They saw service in both World Wars. Revenge fought at Jutland, while Royal Sovereign missed it. In the Second World War, Revenge saw service escorting convoys, while Royal Sovereign was transferred to the Soviet Union as the battleship Arkhangelsk. In 1944, Revenge was disarmed and used as a training ship, while Royal Sovereign would be returned to the UK in 1949. The ships were both scrapped in 1948-49. However, much of the machinery from their gun turrets would be saved. In 1949, Bernard Lovell at the University of Manchester wanted to build a massive radio telescope at Jodrell Bank in Cheshire. This telescope had to be steerable, so they needed two big motors to rotate the telescope in elevation. The turret motors from Royal Sovereign and Revenge were cheap and easy to acquire, given that they were lying around after the battleships were scrapped. The telescope ended up being built around these two motors, and they're still in place in the telescope today.

The oldest building in Virginia actually dates to the late 15th century. Not possible, you say? It was originally constructed in Lancashire, England but was carefully dismantled, shipped, and reconstructed in Richmond in the 1920s. So this English Tudor manor is now a 17th century museum in America.

Since moving to Brussels, I've noticed quite a few surprising uses made of older buildings here. There's a even a verb made up from the city's name that designates buildings whose interior has been exchanged, leaving only the historical facades intact.

A good example of such change is the former hippodrome, the Hippodrome de Boitfort near the forest, which I visited only to find a golf course on the former racecourse. The golf course was founded in 1988 (featuring nine holes), while the hippodrome itself goes back to 1875.

I was also impressed to find the Comic Strip Museum inside a large building by famous Art Nouveau architect Victor Horta. It was designed in 1905 and originally housed the textile department store Magasins Waucquez. The building was left to abandon since the 1920's, and only saved through an initiative started by an architect and a few well-known cartoonists (including Hergé) after 1980. Similarly the current Music Instrument Museum is housed in another Art Nouveau building from 1899 , the former Old England department store.

To finish up I thought I'd mention the world's only streetcar traversing a forest, the tram 44. Its rails were originally built in 1897 to connect two parts of an international exhibition, the palais du Cinquantenaire and the château de Tervueren – all three constructions started by Leopold II. The rails are still in use today, using historical streetcars in summer.

This isn't that interesting, but I have to talk about it because I was there yesterday.

After the USS Nautilus was decommissioned she was painstakingly preserved and turned into a living museum of naval history. Obviously the boat is small so it's built around a whole complex, but the Nautilus is definitely a centerpiece.

One interesting thing about the museum is that it's run directly by the U.S. Navy, which of course has access to the entire naval history of the nation. It's a very educating visit, even for someone like me who knows too much about submarines already.

The Kazan Cathedral in St. Petersburg was transformed into a Museum of the History of Religion and Atheism in the Soviet period. Needless to say, it did not present religion positively. It was transformed back into a church in the post-Soviet period. Now people line up for ages to kiss the Orthodox icon (Our Lady of Kazan) within the church.