The maritime security agenda has diversified and multiplied. Environmental concerns, such as pollution, illegal fishing and climate change adaptation were included in the debate. Most lately, cyber security concerns, and the protection of offshore energy and underwater data cables further enriched the agenda. Over the course of this expansion, also the spaces that concern maritime security have widened. Today security at sea needs to consider a multi-dimensional spatial domain. Maritime security concerns six spatial dimensions: surface, airspace, low orbit, subsea, seabed and cyber. In this contribution I review the six domains, then reflect on what it implies to think maritime security in 6D and why in particular the seabed requires further attention.