First comes the warning, that disembodied voice over the loudspeaker: "Please pay attention. An air raid warning has been issued in the city. Please proceed to the shelter on floor minus two."
Then there is the mosquito-like buzzing of hundreds of Russian drones massing just above the clouds.
Immediately after that, a burst of anti-aircraft fire is heard, distant explosions, and then the ominous sound of ambulance and fire sirens.
This is the grim reality of the night in Kiev and other cities across Ukraine.
These are attack drones that explode on impact.
Today, drones are a common weapon in modern warfare, but their use is not limited to the battlefield.
Across Western Europe, far from Ukraine, they have been spotted flying around airports, military bases and power plants, suspected as part of Russia's "hybrid warfare", with some speculating that they are being used to test the defense capabilities of certain NATO countries that are helping Ukraine.
The recent appearance of drones over Poland, as well as the large number of these aircraft that have been spotted over critical infrastructure in many European countries, including Belgium, Denmark, and in the latest case the Netherlands (above Eindhoven airport), have caused concern in some NATO members.
There is now talk of building a "drone wall" to protect parts of Europe.
But how much is it really necessary?
And, more importantly, how achievable is it?
- War in Ukraine and over the skies of Poland: Combat drones shot down, Russians claim they are not theirs
- Another NATO country reports Russian drone incursion
- New incident in Denmark: Drones spotted over military bases
A wake-up call for Europe
On September 9, about 20 Russian drones flew over Ukraine and entered Polish airspace, causing the closure of four airports.
NATO fighter jets scrambled, several drones were shot down, while others crashed across Poland, their remains scattered across several regions.
One of the largest and most serious violations of NATO airspace since the start of the war in Ukraine was a wake-up call for Europe.
That's why the conversation about possibly building a wall against drones seems even more urgent.
"These recent incursions have really given momentum to these conversations," explains Katja Bego, a senior researcher in the international program.gender security at the British think tank Chatham House (Chatham House).
Drones, or officially unmanned aerial vehicles, have already changed the way warfare is fought.
The killing fields in eastern Ukraine mainly use small, short-range drones, usually about 25 centimeters long, carrying deadly explosive charges.
But they do not currently pose a threat to the rest of Europe.
However, larger drones, some of which can fly more than 1.000 kilometers, are fueling calls for a European anti-drone wall.
Russia previously imported Iranian Shahid-136 drones, but is now producing its own version of the Geran-2.
Some Geranis were among the drones that entered Polish airspace in September.
Some wonder what would happen if Russia sent more than 200 drones one day?
Or, say, 2.000 drones?
How would NATO react and, in fact, could it react at all?
After all, raising fighter jets every time would be too expensive.
"This is neither an effective nor a sensible use of taxpayers' money," says Andre Rogachevski, executive director. Netcompany, a Danish IT company that develops digital systems for European governments.
Watch a video about a Ukrainian florist who makes combat drones for the military
Invasion of mysterious drones
Ukraine has stepped up long-range drone attacks on Russian airports and critical infrastructure such as petrochemical plants, bringing war to ordinary Russians.
They are also in use naval drones - unmanned vessels that can move on or under the surface of the water.
Ukraine used such against the Russian Black Sea Fleet and achieved a devastating effect.
But there is something even more sinister than clearly identifiable drones used by states at open war.
And it's the invasion of mysterious, anonymous drones that have appeared.
- 'A new age of naval warfare': How drone ships are affecting the war in Ukraine
- Drone attack on Moscow: What kind of aircraft were they and how they were able to fool the Russian Air Defense Forces
- Russian cruise missiles destroy Ukrainian cities at a "trivial" price
Sometimes these drones appear in the middle of the night, near European airports, and earlier this month they were spotted above the main airport in Belgium, not far from Brussels.
Similar phenomena have been recorded in Denmark, Norway, Sweden, Germany, Lithuania, and the Netherlands.
Unlike the clearly identifiable Russian attack drones in Ukraine, these "civilian drones" in Western Europe have not been armed with explosives so far.
But because they are launched anonymously, it is difficult to prove where they come from, who activated them, or even whether they are launched from passing ships.
Russia is suspected of doing this, and Western intelligence officials believe that Moscow is using proxies to launch short-range drones from the territory of these countries, to cause chaos, disruptions, and problems.
The Kremlin denies any responsibility.
Belgium is a significant target, as the country is home to NATO headquarters, European Union (EU) institutions, and Euroclear, a financial clearing house that processes international transactions worth trillions of dollars.
There is still debate about whether Europe should unfreeze Russian assets worth around 200 billion euros held in Belgium and use them to help Ukraine.
Is it a coincidence then that mysterious drones have been spotted near the airports in Brussels and Liege, as well as a military base?
The UK's Royal Air Force (RAF) has sent a team of anti-drone experts to help Belgium strengthen its defenses.
However, the mysterious drones are worrisome, both because of the danger they pose to aircraft during takeoff and landing, and because of the possibility of conducting surveillance, especially near military bases and critical infrastructure such as power plants.
- 'Poseidon' and 'Burevestnik': Why Russia is testing unusual nuclear weapons
- The superweapon that Putin claims turns targets into ash and dust
- Russian claims about laser weapons - what is it and how does it work?
Anti-drone wall: Why it's not a magic solution?
The plan to build an anti-drone wall is Europe's response to the threat of cross-border drone incursions launched from Russia.
The wall is described as an integrated, coordinated, multi-layered defense system that, in the initial phase, should extend from the Baltic states to the Black Sea.
It would most likely consist of a combination of radar, sensors, jamming systems, and weapons systems to detect drones, then track and destroy them.
Kaia Kallas, the EU's High Representative for Foreign Affairs and Security Policy, said the new anti-drone system should be fully operational by the end of 2027.
It is no surprise that the countries that want such a system to be established as quickly as possible, including Poland and Finland, are precisely those that are geographically closest to Russia.
Katja Bego believes that such a system is necessary and should have been built long ago.
"It's not just about defending against drones."
"There really isn't enough traditional missile and air defense along the eastern border," he adds.
However, an anti-drone wall is not a magic bullet for air defense, and some doubt that such an idea is feasible.
Robert Tolast, a researcher at the Royal United Services Institute (RUSI), a London-based defence and security think tank, argues that building some "kind of impenetrable wall" is impossible.
Still, he understands why it's being talked about and why it should be tried.
"For the countries bordering Russia - the Baltic states, Poland, and even Germany, because they are of course within the range of these long-range aircraft, it is absolutely necessary to try to build such a system," he says.
"The idea is not to actually build a complete wall or something that is completely impenetrable," agrees Bego.
"That's not really possible - neither because of the length of the border nor because the available technologies are not 100 percent impenetrable."
"It's a combination of systems that we hope could detect different types of drones and stop them."
Stopping Drones: Physical Destruction and Disruption
Fabian Hinc, a researcher at the International Institute for Strategic Studies in London, describes the full range of possibilities for detecting drones.
"You can use acoustic detection: airborne radars that are excellent at detecting low-flying targets; ground-based radars that have short range against low-flying targets but work very well against high-flying targets."
"You can use optical systems, infrared systems - and after detecting the target, there are two options - or disable the threat (soft kill) or to physically destroy it (hard kill)".
Hard kill means the physical destruction of the drone, either by gunfire or missiles.
Soft kill involves disabling the drone, usually by electronic means.
Russia and Ukraine manage to avoid disabling drones on the front line because they equip them with fiber-optic cables tens of kilometers long that unwind during flight, but this is not feasible for aircraft that fly hundreds of kilometers above the borders of other countries.
As for destroying drones, Hinc describes several ways in which this can be done: from surface-to-air missiles to fighter jets and helicopters.
"Lasers can also be used, which could also be useful, but they are not quite the miracle weapons that some people make them out to be," he adds.
Andre Rogačevaski believes that an effective alternative could be blocking and jamming signals.
However, for an anti-drone wall to be effective, it must be capable of responding to a wide range of aerial threats that can all arrive simultaneously.
Watch the video: Ukrainian drone operators and actions on the southern front
Financially controversial issue
As tensions between Europe and Moscow have risen since the beginning of the Russian invasion of Ukraine, so too have other incidents of so-called "hybrid" or "gray zone" warfare blamed on Russia, and mostly denied by Moscow.
Such incidents include cyberattacks, the spread of disinformation, the planting of incendiary devices in cargo holds, surveillance, and sometimes sabotage of undersea cables.
And yet, at a security forum in Bahrain earlier in November, Italian Admiral Giuseppe Cavo Dragone, chairman of NATO's Military Committee, told me that of all NATO's defense needs, air defense is currently the top priority.
The first phases of the anti-drone wall are expected to be activated within a few months, although all the details have not yet been finalized.
Meanwhile, Allied Command Transformation (ACT), headquartered in Norfolk, Virginia, is working on longer-term solutions.
That's a tough challenge.
Tolast says the biggest challenge of the anti-drone wall is the size of the territory it needs to protect.
"A huge system of tactical radars is needed for low-flying drones and larger radars for high-altitude targets, which need to be deployed over thousands of kilometers."
"And we also need cost-effective interceptors and forces that will be ready to respond 24 hours a day."
"The system will never be completely impenetrable, and even if the prices of radar and interceptors continue to fall, it is unlikely that this can be cheap."
Financing is a complex issue.
"That's a really tough question for the defense," Tolast says.
"Even despite the increasing defense spending of European countries, [for funding this wall] there will be a lot of competition from other defense sectors - we need more ships, submarines, and even nuclear weapons, and satellites, too."
"That's why building a wall against drones will be a somewhat financially controversial topic for some."
The wall could be financed by a combination of EU funds, national budgets (especially Eastern European countries), and interest on frozen Russian assets.
Bego says that initially the anti-drone wall was planned only for defense along NATO's eastern flank, but that the concept has been expanded since the EU took a leading role in the project.
"Everyone understands that something has to be done and that there is a need to coordinate this and provide money, but who and how - that is still a matter of debate...
"The more impenetrable you want the system to be, the more expensive it will be."
As for the deadline for activating the wall, Tolast believes that 2027 is a very ambitious plan.
But he adds: "By then, a significantly higher level of protection can certainly be achieved."
Should the problem be cut at the root?
Current events make it even more difficult to build a wall against drones.
Because as soon as new measures are introduced against drones, new forms of drone threats emerge that can circumvent them.
Because of all this, it seems like this is some kind of new arms race.
"The development cycles of technologies in this area are extremely accelerated, especially in wartime conditions," says Josh Burch, co-founder of the British company Gallos Technologies, which invests in security technologies.
"This means that any defense against drones will soon become obsolete, as the aggressor adapts its approach."
"The aggressor observes, adapts, repeats, until he finds a way to break through the defense," he says.
So, is the wrong question being considered then?
Instead of building a wall against drones, is it better to target the bases from which the drones are launched - or as the saying goes: the problem should be cut at the root, rather than dealing with its consequences.
"It's one thing to become more resilient to these attacks, but it would be much better if they didn't happen at all," Bego argues.
"And what that really means is making it much clearer to Russia or whoever is behind these activities that this behavior is crossing the line."
"That there are consequences and that they will bear them."
"That's important and that really should be part of the approach."
But any proposal for NATO to strike Russian targets physically, rather than digitally in cyberspace, would be incredibly risky and would cause escalation.
Since the beginning of the Russian invasion of Ukraine on February 24, 2022, the challenge for NATO, and especially its most powerful member, the United States, has been how to help Ukraine defend itself without drawing NATO into a direct war against Russia.
Building a wall to defend against drones in Europe is one thing, but attacking the places from which they are launched is something else entirely.
Cover photo source: Getty Images, Sketchfab
BBC is in Serbian from now on and on YouTube, follow us HERE.
Follow us on Facebook, Twitter, Instagram, YouTube i Viber. If you have a topic suggestion for us, please contact bbcnasrpskom@bbc.co.uk
- Why drones are so important to the war in Ukraine
- What military leaders can learn from the war in Ukraine
- How Ukraine carried out daring 'Spider Web' operation against Russian bombers
- Turkish Bajraktar drones have become a symbol of Ukrainian resistance
- 'Kill the Russians, score points': Ukrainians introduce a new model of warfare
- Who and how can operate drones in Serbia
Bonus video:
