The spectacular first direct detections of gravitational waves (GW) have opened up hitherto unexplored and extreme regions of the universe. To realize the rich discovery potential of GWs with laser interferometry will require new collaborations and initiatives at the interface of physics and engineering. With this Declaration of Intent our research groups join forces to put forward a coherent research program building on our strengths and centered around six challenges in GW science and engineering. Our objectives include novel precision testing of Einstein’s theory of gravity near black holes and in the early universe, and key advancements on the extremely stringent requirements on the mirrors and their coatings in the interferometer cavities. This will forge a cohesive vibrant Flemish research community in this nascent field, firmly embedded in the global collaborations working towards future observatories, thereby fully integrating Flanders into this exhilarating adventure to unlock the dark side of our universe.