(CNN) -- The opening was so small that CNN's Wolf Blitzer -- no physical giant -- had to bend down to climb inside one of 30 or so Palestinian tunnels from Gaza to southern Israel.
"I guess the tunnel was built for relatively short people, because if you stand up you're going to hit your head," Blitzer said of the almost two-mile concrete corridor about 45 feet underground where he reported from Monday.
His visit, accompanied by the Israeli military, revealed conditions in the network of tunnels below Gaza that are a key issue in the current violent spasm between Hamas militants in Gaza and Israel.
Originally built to avoid Israeli and Egyptian checkpoints into Gaza, the tunnels have been vital supply lines for Palestinians in Gaza. Now the snaking underground routes increasingly get used for attacks in Israel.
Israeli Prime Minister Benjamin Netanyahu launched a ground assault on Gaza this month intended to destroy the tunnel network, and Blitzer went into one of them so see what it was like.
Cooler than the surface
So far down, the tunnel was surprisingly cool inside compared to the hot conditions on the surface, Blitzer noted.
Stooped over, he walked along the concrete-encased pathway with wiring visible in some parts.
The concrete came from Israel, as did the electric power Palestinians send down for lights in some parts, said Lt. Col. Oshik Azouli, deputy commander of Israel's Southern Gaza Brigade.
It took two years or more, progressing a meter or two a day, to extend the tunnel into Israel, according to Azouli.
"It's pretty secure, this concrete," Blitzer said. "They spent a lot of effort building this tunnel."
The tunnel ended near a kibbutz in Israel, Azouli said, adding its purpose was to attack people on the collective agriculture community.
Used for attacks
Later Monday, the Palestinian representative to the United States told Blitzer the tunnels originated to get around the blockade of Gaza, but take on another role in conflict.
"Factions in Gaza are utilizing them to target the Israeli military, especially at times of conflict," Maen Rashid Areikat said, adding: "They want to be creative about how to respond to any attacks. and probably also take initiative."
A series of Hamas attacks via tunnels on Israeli soil escalated Israel's response in the current violence.
The method of attack, in which militants spring out unexpectedly from underground, has raised fears among Israelis living near Gaza.
"They are a strategic danger," Israeli government spokesman Mark Regev said of the tunnels later Monday, telling Blitzer it was "unacceptable" to have Hamas attackers "just pop out of the ground on our side of the frontier to murder and kidnap."
However, Palestinian parliament member Mustafa Barghouti called the tunnels a defensive tactic against repeated Israeli invasions in recent years.
"These tunnels are nothing but a primitive way of trying to defend themselves," he told Blitzer.
Israeli military officials refer to the tunnels as "Lower Gaza."
They aren't new. Hamas used one in 2006 to capture Israeli soldier Gilad Shalit and take him into Gaza. He was held captive for five years until a deal was struck for his release in exchange for more than 1,000 Palestinian prisoners.
During its incursion into Gaza, Israel's military has so far found scores of different access shafts leading to about 30 tunnels. It says it destroyed about half, but expects to find more.
Blitzer said Israel wants to remove the tunnels from the equation before agreeing to a cease-fire with Hamas.
CNN's Jethro Mullen and Martin Savidge contributed to this report, which was written by Tom Cohen in Washington.