The Great Mosque that was never built.

In 1195 construction began on a massive mosque in the city of Rabat. This Mosque was commissioned by Yaqub al-Mansur and designed by Jabir ibn Aflah the architect who designed the Giralda of Seville, it was going to be one of the most bad ass mosques in the world. This was the Hassan mosque, presently known as the Hassan tower

Unfortunately, in 1199 just a short 4 years later Yaqub al-Mansur died and the project lost steam and was never finished, 348 columns had been built, several walls and the minaret at 44m was only about half it's intended height, if it was finished it would have been the tallest mosque in the world.

Conveniently so in the neighbouring city of Casablanca is the actual world's tallest mosque, The Hassan II mosque (of course) at a whopping 210m. However, the Hassan tower is an important historical site for Rabat and also a UNESCO heritage site since 2012.