Palestinian militants have fired hundreds of rockets from the Gaza Strip into Israel during the latest bloodletting, and are believed to have thousands more in stock. Where do all these rockets come from, when Gaza is a tiny sliver of land that has no major manufacturing and is constantly monitored by Israel's military?
National Geographic's December edition has a detailed story and more than a dozen striking photos to explain the answer: The Gazans have built an elaborate network of tunnels under their sandy border with Egypt, and rely on these subterranean passages to smuggle in weapons and a vast array of goods that include construction supplies, cigarettes, livestock and even the occasional lion. Yes, a lion.
The magazine piece was months in the making, and by coincidence, has been published just as the Israeli-Palestinian fighting has thrust Gaza back into the forefront of the news.
The tunnels date to the 1979 peace treaty between Israel and Egypt. Part of the agreement split the city of Rafah half of it went to Egypt, and half went to Gaza, which was under Israeli control at the time.
Israel kept tight restrictions on the Rafah border crossing, which Palestinians circumvented by digging tunnels. The passages sometimes began as holes in a basement on one side of the border and emerged hundreds of yards away, inside a house on the other side of the frontier.
In an attempt to prevent weapons smuggling into Gaza, Israel's military destroyed some 1,700 homes on the Gaza side of the border from 2000 to 2004, a time of constant Israeli-Palestinian fighting.
Israel withdrew from Gaza in 2005, and since then, the Palestinians have built the tunnels far more openly. Hundreds of shafts are visible along the eight-mile border between Gaza and Egypt, covered only by tarpaulin tents.
Despite its withdrawal, Israel has maintained tight limits on Gaza's imports and exports. Basic necessities are permitted, but not much else. Egypt generally allows Gazans to come and go through the crossing, but also restricts goods.
With sky-high unemployment in impoverished Gaza, many young men say they have no alternative but to join the tunnel business, which accounts for an estimated 15,000 jobs.
Building and maintaining the tunnels is tough, dangerous work. The passages are narrow and oppressively hot. Cave-ins causing injury or death are common. Pay is low.
"I was so scared," one tunnel worker, Samir, told National Geographic. "I didn't want to [work in the tunnels], but I had no choice."
Samir joined his brother Yussef in the tunnels in 2008. Last year, Yussef died in a cave-in.
"We always expected something bad to happen," Samir said.
In quieter times, the tunnels were used for avoiding taxes and duties, and smuggling mundane items like cigarettes. Now, they are used to bring in basic goods like cement and other building materials that Israel either bars or restricts.
The Associated Press reported a few years back that the lion and other animals in a small Gaza zoo were imported from Egypt via the tunnels. The National Geographic piece refers to a lion that was "improperly sedated, awoke in the tunnel mid-trip, and tore one of the workers apart."
The militant Islamic group Hamas, which controls Gaza and is currently battling the Israeli military, relies heavily on the tunnel trade for government revenues. Taxes are estimated to bring in as much as $750 million a year, according to National Geographic.
The Palestinians long imported parts to build their own rockets, which have traditionally traveled only a few miles into southern Israel. But in this round of fighting, the Palestinians have fired rockets that landed near Tel Aviv and Jerusalem, more than 40 miles away.
The Israelis say these are Iranian-made rockets that were smuggled into Gaza, presumably through the tunnels.