What aspect of St. Paul's Cathedral is notable besides being London's tallest building until 1962?

The Whispering Gallery of St. Paul's Cathedral is particularly notable for its architectural and acoustic features. It is located beneath the dome's inner curve and is famous for its unique ability to carry sound over a distance. When one person speaks against the wall of the gallery, someone standing at the opposite side can hear them clearly despite the distance. This intriguing design showcases the ingenuity of Sir Christopher Wren, the architect of the cathedral, and provides an intimate experience of the building that highlights its engineering marvel.

While the other choices mention various significant aspects of St. Paul's Cathedral, the Whispering Gallery stands out because it represents a specific and extraordinary feature that enhances the experience of visitors. The stained-glass windows are beautiful and hold historical significance, ancient relics are important to some aspects of religious practice within the cathedral, and the overall size is impressive, but the Whispering Gallery's unique acoustic properties offer a distinctive interaction that encapsulates the cathedral's architectural brilliance.