  The explosions on September 26th, 2022, which damaged the gas pipelines of
Nord Stream 1 and Nord Stream 2, have highlighted the need and urgency of
improving the resilience of Underwater Critical Infrastructures (UCIs).
Comprising gas pipelines and power and communication cables, these connect
countries worldwide and are critical for the global economy and stability. An
attack targeting multiple of such infrastructures simultaneously could
potentially cause significant damage and greatly affect various aspects of
daily life. Due to the increasing number and continuous deployment of UCIs,
existing underwater surveillance solutions, such as Autonomous Underwater
Vehicles (AUVs) or Remotely Operated Vehicles (ROVs), are not adequate enough
to ensure thorough monitoring. We show that the combination of information from
both underwater and above-water surveillance sensors enables achieving
Seabed-to-Space Situational Awareness (S3A), mainly thanks to Artificial
Intelligence (AI) and Information Fusion (IF) methodologies. These are designed
to process immense volumes of information, fused from a variety of sources and
generated from monitoring a very large number of assets on a daily basis. The
learned knowledge can be used to anticipate future behaviors, identify threats,
and determine critical situations concerning UCIs. To illustrate the
capabilities and importance of S3A, we consider three events that occurred in
the second half of 2022: the aforementioned Nord Stream explosions, the cutoff
of the underwater communication cable SHEFA-2 connecting the Shetland Islands
and the UK mainland, and the suspicious activity of a large vessel in the
Adriatic Sea. Specifically, we provide analyses of the available data, from
Automatic Identification System (AIS) and satellite data, integrated with
possible contextual information, e.g., bathymetry, weather conditions, and
human intelligence.
