The Nord Stream gas pipeline, broken by explosions recorded under the Baltic Sea on Monday, are particularly resilient, and one expert said it would take at least 100 kilograms of dynamite to burst them.

“The wall of the steel pipe is 4.1 centimeters and is covered with another 6-11 centimeters of steel reinforced concrete. Each section of the pipe weighs 11 tons and reaches 24-25 tons after the concrete is poured,” explains Bloomberg energy columnist Javier Blas.

Nord Stream 1 consists of two pipeline strands, each of which consists of approximately 100,000 steel pipes encased in concrete, according to Reuters. According to Nord Stream, the permanent internal diameter of pipelines is 1153 meters. The sites are located at a depth of approximately 80-110 meters.

A Swedish seismologist said on Tuesday that he is confident that the seismic activity detected at the Nord Stream gas pipeline leak site in the Baltic Sea was caused by explosions and not earthquakes or landslides.

Björn Lund, a seismologist at the Swedish National Seismic Network at Uppsala University, said seismic data collected by him and Scandinavian colleagues showed that the explosions occurred in water, not in rocks beneath the seabed.

According to Lund, it would have taken explosives equivalent to “at least 100 kg of TNT, but probably more” to damage the pipes.

NATO Secretary General Jens Stoltenberg said on Wednesday that leaks from gas pipelines running through the bottom of the Baltic Sea were the result of acts of “sabotage”.

European Commission chief Ursula von der Leyen said on Tuesday that the Nord Stream gas leak was due to sabotage and warned of the “strongest possible response” if active European energy infrastructure was attacked.

The CIA warned Germany several weeks ago of possible attacks on gas pipelines in the Baltic Sea, German magazine Spiegel reported on Tuesday.