AMNA NAWAZ: In recent weeks, Russia has ramped up its airstrikes on Kharkiv in Eastern Ukraine, a city already facing nonstop bombing since the full-scale invasion more than two years ago.

This latest campaign has left the 1.3 million residents of Ukraine's second largest city worried about what comes next as the war reaches a critical juncture.

My colleagues and I traveled to Kharkiv when we were in Ukraine last week to file this report.

Despite Russia's relentless attempts to derail daily life, the city of Kharkiv carries on.

Less than 20 miles from the Russian border, shops open daily for business, walls are graffitied with local pride, and fluttering Ukrainian flags line the blocks.

Even the metro still runs on time.

But, down here, commuters aren't alone.

Since last September, schoolchildren have been attending class inside Kharkiv's subway tunnels.

It's one of the few places they're safe from Russia's bombing campaign that's only intensified in recent weeks; 6-year-old Yulia is one of nearly 800 kids at this station.

Do you know why you're in school down here in the subway?

YULIA, Student (through translator): Because there is a war.

AMNA NAWAZ: There's a war.

And what does that mean when there's a war?

YULIA (through translator): It's where rockets go boo-doon!

AMNA NAWAZ: Is that what you hear?

You have heard booms?

YULIA (through translator): It's like somebody's playing a drum, but a very loud one.

When they hit something, it's gone.

AMNA NAWAZ: Classrooms below ground are nearly identical to the ones above.

Students build LEGOs at playtime, practice their needlework, hone their English, and solve math problems, sometimes with a little help.

Immediately after Russia's full-scale invasion in February 2022, schools closed and classes went online.

Since then, more than 60 percent of the city's educational facilities have been damaged.

For the youngest students, this school is the only one they have ever known.

Hi.

What's your name?

Ivana.

It's so nice to meet you.

Can you tell me your favorite thing about school?

IVANA, Student: Yes, the blackboard.

AMNA NAWAZ: You like the blackboard?

IVANA: Yes.

AMNA NAWAZ: Your English is so good.

Can you draw us something to take as a memory?

Beautiful.

Tell me, what's this?

Beautiful.

But even below ground, the war above looms, especially for kids with parents on the front lines.

STUDENT (through translator): War is when people are dying, but our military is saving them.

AMNA NAWAZ: What's the first thing that you want to do with your papa when he comes home?

STUDENT (through translator): My papa will come back when it's my birthday, and I just want papa to always be with us.

AMNA NAWAZ: And older students, like 11-year-old sixth grader Lisa, remember well what life was like before.

Do you miss the way school used to be?

LISA, Student: Yes, I miss my school.

(through translator): I want to go back to my old school because it's so much better there.

The subway school is fine, but I want to go back to the school where I left things two years ago.

AMNA NAWAZ: More than 2,000 students go to school at five underground locations funded by the city, with over 500 teachers, psychologists, and medical staff on site.

It's just a fraction of the 56,000 students who remain in Kharkiv, but organizers say it's a start.

And where is this?

Kharkiv city education official Hanna Zaykina says few believed this project was possible.

But the school was built in a month.

HANNA ZAYKINA (Kharkiv Department of Education): Children need socialization, so they feel like they are part of Ukraine, so that they can share their successes, not only through a computer screen, but with each other, holding each other's hands, interacting, facing the reality of war together.

AMNA NAWAZ: Not only did they build these air ducts to pump in fresh air for the kids from outside.

They also soundproofed the facility, so the kids don't hear any explosions, and not even the air sirens.

(SIRENS BLARING) AMNA NAWAZ: Since the first days of war, those sirens have been ringing at a terrible pace, more than five times a day in Kharkiv.

Russian forces reached the city's outskirts after the initial invasion, only to be repelled by Ukrainian troops.

An incessant and unflinching air campaign began, including this March 2022 hit on a Central Kharkiv government building.

Today, it bears a quote from Ukraine's 19th century hero, poet Taras Shevchenko: "Keep fighting.

You are sure to win."

Earlier this month, strikes on residential areas killed at least eight people.

Others knocked out power plants, leading to rolling blackouts for much of the region.

And despite quick cleanup efforts, the city remains scarred.

Once-vibrant blocks are now dotted with boarded-up windows, piles of rubble, and buildings in ruin.

Roman Semenukha is the region's deputy governor.

Why do you think Kharkiv is such a target for the Russians?

ROMAN SEMENUKHA, Deputy Governor, Kharkiv Region (through translator): The goal of the Russians is to destroy the sovereignty of Ukraine.

Kharkiv is one of the pillars of Ukraine.

Putin can't eat the elephant whole.

He wants to cut it into pieces.

His goal is not Kharkiv, not Kyiv.

His goal is to destroy Ukrainian sovereignty.

AMNA NAWAZ: The greatest fear among people here and of analysts around the world is that Russia's ramped-up attacks mean a ground invasion is imminent, an invasion that would force people like 20-year-old Lisa Nikonorova to flee their homes for a second time.

LISA NIKONOROVA, Kharkiv Resident: If Russians come here, you have no other choice but to run, because I don't want to be under Russia.

AMNA NAWAZ: Her family went to Western Ukraine in the war's early days, but returned a year later.

LISA NIKONOROVA: This is the only place where I can feel at home.

So, if I move even to another city, I won't feel safe, because this is the place where I belong.

AMNA NAWAZ: That sense of belonging, Hanna Zaykina says, is how the city will survive.

HANNA ZAYKINA (through translator): The only emotion that's unchanged is a great love of Kharkiv.

All the people of Kharkiv are madly in love with it.

My family and I never left the city.

We were educating from day one of the invasion.

When people, when children, were living in the metro stations, teachers didn't leave their kids.

Even though we have lost buildings, we haven't lost a single child's potential.

AMNA NAWAZ: Even in war and uncertainty, down here, there is joy.

I ask Yulia what's her favorite thing about school.

She answers quickly, and with a smile: "Everything."