The entrance is a deep vertical shaft, usually hidden in a house. It drops down a dozen metres or so before reaching a horizontal passage, lined with concrete and electric cables.
Most are around a metre wide and perhaps 2.5m high, barely enough to accommodate a man carrying a heavy load of weaponry.
The tunnels descend deeper, reaching up to 30m below the surface.
Most are between 1 and 3km long and have multiple entrances and branches. They interconnect with other passages and with bunkers used as command centres and weapons stores and to keep Hamas' political and military leaders safe from the pounding by Israeli forces above ground.
This is "lower Gaza" and Israel's casus belli: a secret labyrinth of tunnels and bunkers, painstakingly built by Hamas over recent years at enormous cost.
As Israeli forces race to find and destroy as many cross-border tunnels as possible, Hamas and other militant groups are using their underground strategic weapon to launch attacks against troops, both within Gaza and across the border in Israel.
On Friday, militants emerged from a tunnel near Rafah, in the south of Gaza, to kill two soldiers and apparently abduct a third, although Hamas later claimed he had probably died in a subsequent Israeli air strike. Earlier last week, five Israeli soldiers were killed at an Israeli military watchtower by militants who had crossed the border underground and emerged through a hidden tunnel shaft.
The extent and sophistication of the underground network has taken Israel's political and military leadership by surprise. Nevertheless, more than two weeks after the Israel Defence Forces (IDF) launched a ground offensive in Gaza, it says it is close to achieving its goal. "We are just a few days away from destroying all of the offensive tunnels," the IDF's southern command chief, Major-General Sami Turgeman, said.
Three different kinds of tunnels exist beneath Gaza, said Eado Hecht, an Israeli defence analyst specialising in underground warfare: smuggling tunnels between Gaza and Egypt; defensive tunnels inside Gaza, used for command centres and weapons storage; and - connected to the defensive tunnels - offensive tunnels used for cross-border attacks on Israel. The military says it has located about 32 to 35 offensive tunnels, of which more than half have been destroyed, and it believes that there are around 40 in total.
Tunnel construction began in Gaza more than a decade ago; IDF soldier Gilad Shalit was abducted by militants on the Israeli side of the border and dragged back into the enclave in 2006. But the industry took off after Israel tightened its blockade of Gaza the following year. Hundreds of tunnels for smuggling goods and people were built under the Gaza-Egypt border; Hamas also dug separate tunnels for bringing weapons into Gaza.
In the past few years, Hamas has used the valuable expertise acquired in the construction of the smuggling routes to build a network of defensive and offensive tunnels. The offensive tunnels have been dug by hand, as the use of machinery would risk detection. Military analysts estimate that each tunnel takes two to three years to complete, and costs millions of dollars.
Israel has said that the "demilitarisation" of Hamas and other militant groups in Gaza is an essential element of any agreed truce to end the current conflict.
However, Brigadier-General Shimon Daniel, head of the IDF's combat engineering corps from 2003 until 2007 and now a reservist, said Hamas would not easily be deterred. "Of course Hamas will try to rebuild the tunnels. The moment we go out [of Gaza] they will begin to dig."