The vessel is part of an exercise bringing together six countries and more than 20 companies and researchers hoping to stay ahead of the enemy as the danger to Europe's critical underwater infrastructure rises.

From gliders flying low over the sea surface to detect anomalies to a "crawler" deployed on the seabed to remove mines, the vessels are tackling a variety of challenges in a testing seabed measuring 10 square nautical miles.

The possible sabotage of undersea cables has hit the headlines in recent years due to a series of incidents, with the finger often pointed at Russia and China.

The most recent of these came in December, when the EstLink 2 electricity cable and four telecoms cables that lie on the seafloor linking Finland and Estonia went offline after suspected sabotage.

Suspicion fell on the Eagle S, an oil tanker flying the Cook Islands flag but thought to be part of Russia's so-called "shadow fleet" -- ships that carry Russian crude oil and petroleum products embargoed due to Moscow's invasion of Ukraine.

"It's not a question any more of if it will happen. The question is when the conflict will start," said Rear Admiral Paul Flos from the Dutch navy.

"We have to be ready for it. And that's what we're doing here today," added Flos in an interview with AFP.

He said attacks by Russia and China on Europe's undersea infrastructure were "absolutely increasing" and the lessons learned at the testing centre were helping to counter the threat.

The systems were being challenged to detect another vessel snooping around a pipeline, spot a tiny mine laid beside a cable or notice something on the seabed that should not be there.

The advantage of the test centre in Scheveningen is that visibility is very poor and the seabed is extremely sandy, meaning the conditions are harsher than in the North Sea.

"If it works here, it works everywhere," said Flos, 58.

"At the moment, we're blind. And with what we're doing today... we are trying to find out what kind of equipment can best support us and to make sure that we're not blind any more," said Flos.

- 'The effect is huge' -

Another high-profile incident came in September 2022 when the Nord Stream natural gas links, which run along the Baltic seabed between Russia and Germany, were partially severed.

A field of bubbles formed on the surface above the pipelines as gas flowed out.

Seismic records later indicated there had been a series of underwater explosions just before the leak was discovered.

European officials declared it an act of sabotage and blamed Russia but more recent media reports have linked the incident to Ukraine -- an accusation Kyiv has strenuously denied.

In response to the growing threat, NATO hastily pulled together the Baltic Sentry patrol mission early this year.

The laying and operation of underwater cables was long the preserve of large telecoms operators but the internet giants have largely taken over in recent years, as they strive to keep up with ballooning flows of data.

About 1.4 million kilometres (nearly 900,000 miles) of fibre-optic cables are laid on the ocean floor, enabling the provision of essential services such as trade, financial transactions and public services around the world.

The impact of a major attack on Europe's critical infrastructure could be devastating, said Carine van Bentum, head of the SeaSEC (Seabed security experimentation centre) testing hub.

A country could be brought to a "complete standstill", the 48-year-old told AFP in an interview.

"If we do not have power, we as a society are not resilient anymore. We have no idea what to do. If we do not have internet, we cannot pay. So the effect is huge."