JERUSALEM -- The Israeli military slowly lowers our crew underground using repurposed climbing equipment.

Inside a steel tube, looking down hundreds of feet, the entrance to a Hezbollah attack tunnel is seen.

The air is thin and it’s noticeably hotter as we progress deeper into the structure, at one point the Lebanese border is just a few feet away.

In January, the Israeli Defense Forces uncovered six Hezbollah attack tunnels from Lebanon into Israel during an operation they called “Northern Shield.”

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“One of our most important responsibilities is to keep an ongoing battle against tunnels both in the Southern and Northern frontier,” An IDF officer told Fox News.

An Iranian inspector previously visited the first tunnel we entered, examining the work of Iranian-backed Hezbollah fighters. The group is estimated to have more than 25,000 active soldiers.

The largest tunnel started in a civilian home in the Lebanese village of Ramiyeh. It was more than a kilometer in length and nearly 260 feet underground.

While a simple core drill was used to dig the tunnel, along with a few smaller tools, some of the structures were equipped with telephone lines, a rail system to transport goods and electricity.

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Amid new regional tension with Iran, reports indicate these tunnels were meant to be used by the Iranian proxy Hezbollah in a ground operation against Israel.

Hezbollah leader Hassan Nasrallah confirmed they were supposed to be used as a tool for war.

“If there is a war, and I say if we decide to enter the Galilee, the tunnels will partially help. An operation like this one needs the whole border area. I say to them: ‘You will never know how we will go in; on the ground, underground from the sky,’” Nasrallah said.

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Following our visit, the Israeli military destroyed most of the tunnels to prevent Hezbollah militants from reusing them.

With this threat addressed, Hezbollah still poses a very real threat. The group is estimated to have more than 100,000 rockets in their arsenal.