After scrabbling up a slope in this desolate valley amid Lebanon's craggy southern hills, I found it: an ominous pitch-black hole partially blocked by a layer of rock.
It would be a tight squeeze to get in. And going farther was potentially risky.
Our discovery was so rare and revealing that it could have been booby-trapped with explosives. I checked for tripwire, but didn't see any.
"Found it. It's open. We can get in," I called to my two colleagues, laboring up the hill.
We were about to enter the secret world of Hizbullah, the militant Shiite group that battled Israel from this perch, and dozens of other hidden positions, last summer. We weren't sure what we'd find below, but were certain it would tell us a great deal about the capabilities of the Lebanese guerrillas that fought from these steep limestone hills covered in a dense undergrowth of scrub oak and juniper bushes.
Pausing to catch my breath, I shrugged off my backpack and reached inside for a head lamp.
As we climbed in, air chilled by the deep subterranean passageways wafted out of the entrance, a refreshing contrast to the blazing heat of the valley.
I had been hunting for one of Hizbullah's bunkers since the end of the 34-day war.
It had been a frustrating exercise, to be sure. The bunkers and rocket-firing positions had been constructed in great secrecy, the entrances cunningly camouflaged, in remote valleys along the Lebanon-Israeli border.
In addition to possible booby traps, cluster bombs, and other unexploded ordnance litter many of Hizbullah's abandoned "security zones" in valleys and hilltops along the border.
In March, I was fortunate enough to have received map coordinates from a source that led me to a bunker, which could be accessed by a 20-foot shaft.
A second series of map coordinates, which I tapped into a global-positioning system (GPS) device, led us to this spot about two miles north of the Israeli border near Rshaf, earlier this week.
As we followed the arrow on the GPS, we could hear the whine of an Israeli reconnaissance drone, invisible against the brilliant blue sky, as it slowly circled high above us. It was probably searching for signs of new Hizbullah activity.
Shining my head lamp into the entrance, I could see that the pile of boulders only ran for a few feet, after which the opening widened into a passageway.
The walls and ceiling were reinforced with steel plates and girders painted black to prevent stray reflections from the sun giving away the concealed entrance.
As I crawled in the tunnel, I watched carefully for scorpions and spiders. The passage ran horizontally for about 10 yards before doglegging to the right.
It was little more than shoulder-width, and we had to stoop slightly to avoid hitting the ceiling with our heads. Once around the corner, the steel plates were painted white, this time to better reflect the electric lighting.
Electric cables ran through white plastic tubes, fixed to the walls, leading to switches and glass-encased light sockets. A blue plastic hose running along the top of the wall carried the bunker's water supply.
The first room we encountered was a small bathroom complete with an Arab-style latrine, a shower, a basin with taps, and a hot water boiler. There was even a drainage system constructed beneath the concrete floor.
The air was blissfully cool after the sun-drenched heat of the valley. In two places along the main passage – which must have been more than 60 yards long – were vertical ventilation shafts covered by metal grills, ensuring a steady flow of fresh air.
We were perhaps 100 to 150 feet underground at this point, deep enough to withstand almost anything in Israel's arsenal. I let my colleagues walk on and then switched off my head lamp.
The sudden darkness and utter silence was unbearably oppressive.
What must it have been like for the dozen or so fighters housed in this bunker, awaiting the advancing Israeli troops?
There was a kitchen with storage shelves and an aluminum sink and taps. The white metal walls were mottled with brown rust. Every 10 yards or so along the passage was a heavy steel blast door that could be locked from the inside with a bolt.
As far as I know, this is the largest and most elaborate bunker discovered so far.
Just the effort that went into building it was extraordinary, and yet, it was constructed in complete secrecy.
Most likely, no one outside Hizbullah knew it existed until two weeks ago, even with peacekeepers from the UN force known as UNIFIL (UN Interim Forces in Lebanon) patrolling the ground and Israeli aircraft watching from the skies above.
Every piece of equipment, every steel plate, every girder, every door had to be carried by hand up the side of the valley and fitted into place inside the bunker.
And there was no clue as to what happened to the hundreds of tons of quarried rock during the excavation work.
Six years of building
While it was widely suspected that Hizbullah had been building underground facilities in the six years prior to the war, it was only after the Aug. 14 cease-fire that their scale and sophistication was understood.
Israel had seriously underestimated its foe and Israeli Prime Minister Ehud Olmert and other top officials are fighting for their political survival as a result.
"It was a combination of a monumental intelligence failure – the Israelis only found these bunkers by stepping on them – and extremely professional and efficient work by Hizbullah," says Timur Goksel, a Beirut-based consultant on Mideast security issues and a former senior adviser to UNIFIL.
Now, the bunkers are useless. Their locations having been compromised.
Hizbullah has abandoned all the bunkers in the UNIFIL-patrolled zone along the border, redeploying to a newly constructed line of defense farther north.
In this bunker, only a green sleeping mat and a simple metal bed frame remained. At the far end of the bunker, the narrow steel-lined passage broadened out into a rock cavern.
In a niche to one side were four metal water tanks with the Arabic word for "sacrifice" painted across them. A twist of a tap at the bottom of one tank, and icy water gushed out. Several steep steps cut into the rock at the end of the cavern led to an access shaft about 15 feet high with a ladder soldered onto the lining of black metal plates.
Climbing up led us back outside into a thicket of stubby oak trees about 40 yards from the entrance and farther up the hill. The Israeli drone still prowled overhead, its cameras perhaps hunting for the three mysterious people who had suddenly disappeared into thin air on the hill.