The Iran-backed movement has exchanged regular fire with Israel in support of its ally Hamas since the Palestinian militant group's October 7 attack on Israel sparked the Gaza war.
Prospects of full-blown conflict grew after Iran and Hezbollah vowed revenge for the killing last month, blamed on Israel, of Hamas leader Ismail Haniyeh in Tehran, and an Israeli strike that killed Fuad Shukr, a top Hezbollah commander, in south Beirut.
Nicholas Blanford, a Beirut-based Hezbollah expert and senior fellow at the Atlantic Council, said a Hezbollah video released Friday showing underground tunnels and missile launchers could be a "warning" to Israel.
AFP could not verify the video's authenticity.
Hany Farid, a digital forensics expert at the University of California, Berkeley, said it was "unlikely" to have been generated by artificial intelligence.
But parts of the video "might incorporate classic CGI gaming footage", he added, referring to computer-generated imagery.
Here's what we know about the footage and Hezbollah's tunnels.
What tunnel video?
Hezbollah's polished, four-and-a-half minute video titled "Our mountains are our storehouses" showed what appeared to be underground tunnels big enough to fit convoys of trucks.
Some trucks appeared to transport missiles and launchers through the facility, identified as "Imad 4" -- a reference to top Hezbollah commander Imad Mughniyeh, killed in a 2008 Damascus car bombing blamed on Israel.
"Some of the footage as the trucks/motorbikes move through the tunnel have a slight CGI look to them," Farid said, adding he could not say for sure whether the technology was used.
Blanford said that "in the context of Hezbollah's expected retaliation" to Shukr's killing, Hezbollah "probably wanted to remind" Israel that it can "unleash far more powerful weaponry" should Israel's counter-attack be too strong.
For Lebanese retired brigadier-general Mounir Shehadeh, the video showed "how deep, how large and how complex (the tunnels) are, and how difficult or even impossible it would be for Israel to reach them".
What are the tunnels' advantages?
Hezbollah's backer Iran also has underground facilities.
"We call the underground missile facilities... 'missile cities'," Tehran's embassy in Beirut said Friday on X, adding that they "exist all over Iran" and allow forces to "strike the enemy from anywhere" in the country.
Iran's Mehr news agency said Saturday that the Hezbollah video showed a "missile city under the Jabal Amel mountains", a term commonly used to refer to south Lebanon where, alongside east Lebanon, Hezbollah has a strong presence.
Military analyst Hisham Jaber, a retired Lebanese general, said little was known about Hezbollah's "top secret" underground bunkers and tunnels.
The "Imad 4" facility is probably one of dozens, he said, adding that "south Lebanon's mountains and hilltops are ideal for digging (facilities) that are protected because they are at the heart of a mountain".
"Warplanes cannot reach these facilities," Jaber told AFP, and fighters could remain inside well-provisioned tunnels for months.
Israel could "keep on destroying Lebanon for months without ever reaching" the bunkers, he added.
Orna Mizrahi, a Hezbollah expert at the Tel Aviv-based Institute for National Security Studies, said Israel has known about the underground facilities "for a while" and has experience dealing with Hamas tunnels in Gaza.
"We have good experience with what's going on in Gaza and then I suppose that this is what we'll have to tackle if we are going inside Lebanon in the next war," she told AFP's Jerusalem bureau.
What's the tunnels' history?
At a Hezbollah "tourist complex" in Mlita in south Lebanon inaugurated in 2010, a tunnel stretching 200 metres (220 yards) belonging to the group is on display.
Blanford said he believed Hezbollah's tunnel networks began in the mid-1980s when Israeli troops withdrew from most of Lebanon to an occupied strip along the southern border.
"It's been widely understood for a long time that Hezbollah has extensive tunnel networks... used to store munitions and to serve as low-signature missile (and) rocket launch pads," he said.
Tunnels built in the early 2000s resembling the one in Mlita were "designed for a small number of fighters to rest, sleep and eat", said Blanford, while the facility in Friday's video "completely dwarfs those earlier man-sized bunkers".
Daniel Meier, head of the Middle East masters programme at Sciences Po Grenoble, said Hezbollah's tunnel usage during its 2006 war with Israel, especially in the border town of Bint Jbeil, put heavy pressure on Israel "despite its air supremacy".
After that, Hezbollah began building more complex underground facilities and tunnels, experts told AFP.
In January 2019, the Israel military said it uncovered and destroyed "cross-border attack tunnels", some burrowed under Lebanese border villages that Israel has repeatedly struck since October.
