Flandres is the northern part of Belgium, and is Flemish speaking. This language is similar to Dutch.
Although not much smaller than Wallonie there are almost no subterranean sights. The reason is easy: this area is mainly lowlands with high ground water table. Part of the area along the coast is tideland. This area is not karstified and there are few mines. At the moment there are not even subterranea listed for this area.