Israel's goal in launching a ground invasion into Gaza is to secure a swath of land inside the territory's perimeter fence to prevent access to tunnels that could be used for attacks on Israel and to impede the launch of short-range rockets.
Five tunnels had been located by Israeli troops by lunchtime on Friday and were under investigation, the Israel Defence Forces said.
In an operation that could last at least a week, Israel wants to expand significantly the existing buffer zone. The no-go area currently runs 500 metres (1,600ft) into Palestinian territory along most of the Israel border, but Israel could extend it to as far as 3km (1.86 miles) – a huge proportion of such a tiny strip of land. Most of the area is currently agricultural land, but this expansion would take the buffer zone into urban areas of Gaza.
The military's primary aim is to locate and destroy tunnels dug by Hamas and other militant groups from Gaza into Israel. It says the tunnel network is "highly developed and continuously preserved". The number of tunnels is unknown, but Ma Gen Gadi Shamni, a former commander of the IDF's Gaza division, said the Israeli military assumed there were dozens. He said the existence of tunnels penetrating Israel was a "major threat that we have to address".
On Thursday, Israeli troops foiled an attempted attack by militants from Gaza who emerged from a tunnel close to Kibbutz Sufa, near the border. Thirteen fighters armed with rocket propelled grenades (RPGs) and assault rifles were seen by an IDF spotter who called in an air strike, the military said. Lt Col Peter Lerner, an army spokesman, said the interception had "foiled a terror attack that could have had devastating and deadly consequences".
The Israeli prime minister, Binyamin Netanyahu, said the ground invasion was necessary to destroy tunnels. "It is not possible to deal with tunnels only from the air. It needs to be done also from the ground," he said before a special cabinet meeting in Tel Aviv on Friday.
Hamas and other Palestinian organisations – both militant groups and business entrepreneurs – have developed expertise in digging hundreds of tunnels during the past seven years of blockade on Gaza. The vast majority of tunnels have been dug from Rafah, at the southern end of Gaza, to Egypt, and have been used to smuggle goods, fuel, construction material and people. Militant groups dug tunnels to smuggle weapons. Since the military coup in Egypt a year ago, almost all the tunnels from Rafah have been destroyed.
There are far fewer tunnels dug for military attacks on Israel – although the Israeli soldier Gilad Shalit was captured after militants emerged from a tunnel on the other side of the border. Shalit was dragged back to Gaza through the tunnel, and was held for five years before his release in 2011 in a prisoner swap.
Last year, the IDF uncovered a tunnel from Gaza to Israel that was 59ft (18 metres) underground, extended for a mile (1.6km) and was constructed using 500 tonnes of concrete and cement.
Fresh tunnels have been dug between Gaza and Israel in the past two years, said Kobi Michael, a senior fellow at the Institute of National Strategic Studies and a former head of the Palestinian desk at the ministry of strategic affairs.
He added: "Hamas understands that tunnels are a strategic weapon against Israel. They have learned that the actual impact of rockets is less dramatic than they expected, so the other options is tunnels."
Hamas had devoted immense resources to building tunnels, Michael said: "They are very high quality, very deep, very long, well structured, very professional."
Construction of the tunnels – many of which are connected to an electricity supply, and some to telephone lines – can take years. The Israeli military has used geothermal imaging and other hi-tech methods to try to locate them.
The current Israeli operation to destroy these "terror tunnels" is complex and risky. It involves troops searching on open ground while being given cover from their air and from tanks.
According to Shamdi, many of the tunnels have offshoots in different directions, "so it's very hard to track the exact route". Even if a tunnel shaft is destroyed, it would be relatively easy for militants to dig another entrance to an offshoot, he said. "This is a great operational challenge. The chances at the end of this operation that we locate 100% of the tunnels is not high."
As well as cross-border tunnels, Hamas has also built a series of interlinked underground passages and bunkers within Gaza, in which its senior leaders are currently protected from bombing.
Hugh Lovatt, Israel-Palestine coordinator at the European Council on Foreign Relations thinktank, said: "Should the IDF advance deeper into Gaza and into heavily populated areas these tunnels will also give Hamas a tactical edge, not only shielding them from air strikes but also allowing Hamas fighters to mount ambushes and harass advancing Israeli soldiers."
Guerilla fighters in the past conflicts had used tunnels "to mount lightening raids against more conventional forces and then melt away before the opposing force can bring its full military might to bear. Examples include Stalingrad, Vietnam and also southern Lebanon in 2006 where the IDF encountered a series of well-prepared Hezbollah defensive positions which caused significant casualties amongst IDF ranks."
Hamas said on Friday that the IDF would pay a high price for its ground invasion. "Gaza will be the graveyard of the occupation soldiers, and these are not slogans," a Hamas spokesman, Sami Abu Zuhri, told Lebanese television. Hamas and other militant groups in Gaza are likely to attempt to draw Israeli troops into fighting on the ground in urban areas. The IDF, which had its first military casualty overnight, would be exposed to a much higher risk of death and injury.
Israel also risks getting sucked into an undesirable re-occupation of Gaza, nine years after withdrawing troops and settlers from the territory. Indications are that Netanyahu and his close colleagues are anxious to restrict the ground operation to the goal of clearing and securing land close to the border rather retaking control of the entire enclave.
Michael said that "mission creep" was possible in the current operation. "As far as I understand, the end objective of the military is limited, but it might not end only with tunnels."
The extension of the buffer zone was "not only a question of the number of metres from the fence, it's also about the nature of control". Israel might demand that the zone be policed by Palestinian Authority or even Egyptian forces, he said. "And Israel will want the ability to act there if necessary. The circumstances of Palestinian civilians who live and work near the fence will be much less convenient."