The center of tourist attractions in Istanbul is Sultanahmet Square. On one side you have Hagia Sophia, on the other the Blue Mosque, and right beside that the Hippodrome, all a hop and skip away from the majestic Topkapi Palace. What you don’t see - and I walked past it on my first visit to Istanbul - is that right next to Hagia Sophia: an underground forest of Roman columns called Basilica Cisterna.