After the 2004 and 2005 train bombings in Madrid and London, Calatrava was forced to double the number of steel ribs on the Oculus to improve blast resistance. Soon after, New York City officials turned down his proposal for skylights that would have been built into the pavement of the neighboring National September 11 Memorial in order to draw natural light into the hub's subterranean passageways. Trying to curb soaring costs, Port Authority officials canceled plans for the hub's wings to move, as they do in Milwaukee. Rising construction costs, spurred by Manhattan's building boom, added to the budget bloat.