Researchers Barbora Wouters (VUB) and Yannick Devos (ULB) have examined the black layers of both cities. These layers are situated in the dark soil levels near a city's oldest remains.
Normally, archaeologists search with spades and brushes, but Wouters and Devos used a more subtle manner, using a microscope. This showed traces of habitation or human activity before the first city walls were erected, mainly agricultural activities before the area turned into a city.
For Antwerp, this is before 900. Wouters assumes that 9th- and 10th-century Antwerp was in contact with the Vikings. Devos focused on Brussels, and also discovered traces of farming and cattle keeping from before the city walls were built.