Updated October 3, 2022 at 9:37 AM ET

A week after several suspicious ruptures were discovered along the undersea Nord Stream pipeline, gas has reportedly stopped leaking but the questions keep flowing. Namely: What — and who — caused the damage? And how can countries try to prevent similar incidents going forward?

Scientists say the two leaks were likely caused by powerful underwater explosions, based on seismic data from Sweden and Denmark. And European leaders believe those leaks were no accident, with NATO blaming "deliberate, reckless, and irresponsible acts of sabotage."

Sweden, Denmark and Germany have all opened investigations, while Russia — whose state-controlled energy company, Gazprom, is the main owner of the pipelines — is calling for a review and blaming the West.

Neither pipeline was operational at the time of the leaks, since Moscow cut flows to Nord Stream 1 in August (in retaliation for Europe's support of Ukraine and sanctions in Russia) and Germany postponed the opening of Nord Stream 2 just before Russia invaded Ukraine in February.

Still, the incident has raised a number of concerns about the impact on the environment (because of the massive amount of climate-damaging methane it released) and Europe's energy supply, as well as the vulnerability of critical infrastructure more broadly.

The Nord Stream leaks serve as a warning that any one of the numerous gas, power and telecommunication cables crisscrossing Europe could be a target, as NPR's Jackie Northam put it. Taking note, countries like Norway, Denmark, Italy and Poland have ramped up security and surveillance around their own undersea pipelines in recent days.

There are about 730,000 miles of pipelines around the world, enough to circle the Earth 30 times, says national security analyst Juliette Kayyem. And that's not including wires connecting the internet, which she says amount to "another million miles of cable, essentially."

These pipelines are "lifelines for these countries to continue moving and living and having access to electricity," Kayyem told Morning Edition's A Martínez on Monday. "They're vulnerable because they're large, they are exposed — at least under the ocean floor — and they are very hard to protect."

What steps can countries take to protect their critical infrastructure?

That doesn't mean no one is trying to protect these vulnerable pipelines, Kayyem says.

"We should think about it more and people do think about it," she explains. "The standard is not, 'Can we make it safe?' It's just sort of, 'Can we make it safer?' at this stage, can we minimize the risk for these pipelines."

There are four primary ways that security experts think about minimizing risk, according to Kayyem, who describes the approach as "layered defenses."

It involves building more protective pipes that can withstand intrusion, controlling access to pipelines, monitoring them and intercepting potential attacks.

Kayyem says the Nord Stream incident shows the importance of surveillance, and making sure countries know which boats and submarines are around the pipeline. These kinds of pipelines have lots of detection devices, she adds, which can help authorities figure out if the object nearby is a whale or an actual threat.

And if something does look like it's attacking the pipeline, are a country's forces prepared to go after it? Kayyem says they could use battleships, submarines or even equipment like drones to "take things out."

"It's a little bit like warfare, because you are protecting something that would be attacked in a war," she adds.

Not all threats come by land or sea. It was just last spring when a ransomware attack temporarily shut down the Colonial Pipeline, which carries gas, diesel and jet fuel along the U.S. East Coast from Texas to New York.

Kayyem says it's crucial for countries and companies to plan for these kinds of disruptions so they can prepare to restore their systems as fast as possible.

"We just have to anticipate that there will be attacks," she says. "And then the standard of success is, can we get back online quickly."

The audio interview was produced by Naina Badarinath Rao and Julie Depenbrock, and edited by Simone Popperl and Reena Advani.

Copyright 2023 NPR. To see more, visit https://www.npr.org.