Nope, it’s not St. Patrick’s Cathedral.
When you enter the Basilica of the National Shrine of the Immaculate Conception on the campus of the Catholic University of America, you have the impression that you are not in the US, but in Ravenna, Italy. Dedicated to the patroness of the United States, this basilica located in the capital of the country, Washington, DC, is also the largest Catholic church in the United States, and one of the ten largest Catholic churches in the whole world.
Construction of the church began in 1920, although since the birth of the United States as a nation independent of the British Empire the country had been consecrated to the Immaculate Conception. Moreover, from about 1792 onward, the construction of a national church dedicated to this Marian patronage was already dreamed of. But it was not until 1913 when the fourth president of the Catholic University of America formally proposed the construction of the basilica to Pope Pius X. And so, the first funds for construction were finally received.
Although the first Mass was celebrated in the basilica back in 1959, the building is not is still not finished. Work continues, as it did on the great medieval cathedrals, over the decades. At present the splendid mosaic work under the Trinity dome is being completed. You can appreciate the beauty of the basilica’s existing mosaics in this slideshow: