Inside Titanic’s foremast.
This is a view (render) from the top of the ladder leading to the Crow’s Nest. From here, you can see the floor of the Crow’s Nest, and the bottom of the mast interior on C-Deck. It was through a door on C-Deck that lookouts could access the mast ladder and Crow’s Nest. It was in that very spot on the upper-right the lookouts Frederick Fleet and Reginald Lee were standing when they spotted the iceberg.