The Flexible Underpasses (FLUPs) are modular underground network puzzle pieces that enable you to build tunnels around corners or modular underpasses (with similar flexibility as the overpass puzzle pieces). The model of the Marrast's Road Underpass is used as ramp, but unlike the static Marrast underpass lots, FLUPs are flexible and modular and allow more complex underpasses. The first set of FLUPs, for the Road network, was included in the January 2009 NAM, and Avenue and One-Way Road FLUPs pieces were introduced in the June 2009 NAM. The idea for these puzzle pieces also came from the rail underground puzzle pieces inside the NAM. The project is led by Chrisim.