When I was very young, probably six or seven, I thought the D.C. in Washington D.C. stood for "Decorated by Columbus".
I imagined Columbus running about ballet style flinging columns and buildings and painting the sky patriotic. Oddly, I never thought it was "Discovered by Columbus", probably because that wouldn't lead to such entertaining images. ;)
A childhood belief about places

others believed things like this
- I thought that Spandu Ballet were ballet dancers and wore tutus.— Helen
- When I was little I thought that Washington( D.C. )was in Washington state. Anon
- I used to believe that the lower level of the George Washington Bridge in NYC was named Martha Washington— Shayne
- I used to believe that the oddly-shaped clouds were made from oddly-shaped sheep— Dickson
- I once thought people would keep growing until they were as big as buildings if they didn't go to the doctor. — Tufty
- I thought the "To Let" signs on buildings meant they were public toilets.— Scottish Laddie