* Tunnel search a main mission of Israeli ground operation
* Palestinian guerrillas see tunnels as strategic assets
* Israelis map out access shafts, then blow them up (Adds background, details on Israeli soldiers killed in tunnel shaft)
By Dan Williams
GAZA STRIP, July 30 (Reuters) - Israeli Colonel Tomer was literally tipped off to the tunnel snaking under a Palestinian village when the tank-churned earth gave way to the weight of one of his behemoth bulldozers' treads.
Twenty-four hours later, on Wednesday, his forces had cleared a greenhouse that had provided cover and dug a three-metre-deep (10 foot) crater exposing the concrete-reinforced passage wide enough for a man in battle gear to squeeze through.
It would take several days, the officer told Reuters at the scene - the exact location could not be reported under military rules - to map out the half-dozen suspected access shafts to the tunnel, one of which, he said, was concealed by a nearby home.
Then explosives would be dropped in and the network destroyed as part of a tunnel-hunt throughout the Gaza Strip's eastern frontier that Prime Minister Benjamin Netanyahu says will be completed, whether or not Israel reaches a ceasefire with Gaza's Hamas Islamists.
Tomer's soldiers, a mix of tank crewmen and army engineers, watched the dark, descending void of the tunnel aperture with assault rifles raised, on guard for any gunmen that might storm out. An electric whiz overhead signalled a Palestinian sniper shot gone wide; Israeli tank machine-guns chattered back.
At a different location, in southern Gaza, three Israeli soldiers died on Wednesday when a booby-trapped tunnel shaft they had uncovered blew up, the army said. Referring to such casualties, Tomer said his forces were keeping a safer distance.
"We don't put men in the tunnels. There is too much risk of booby traps," Tomer, who under military rules could be identified only by his first name, told a Reuters correspondent invited by the army to view his force's operations.
Instead, he said, the army sends in dogs to sniff out explosives and robots that relay back video images through which the layout can be learned.
So guided, an earth-mover digs into the next stretch, where the process is repeated, to continue until the entire 1.5 km (one mile) stretch to the Gaza-Israel border is covered.
Were the troops to track the tunnel in the other direction they would end up 2 km (1.2 miles) deeper in Gaza, in the middle of a bustling neighbourhood, Tomer said. But that route, on the opposite side of the crater, was blocked with sand.
"We don't want to go there. What concerns us is the stretch headed toward Israel," he said.
The hunt is personal for Tomer, who lost one of his officers and a conscript in a July 19 clash with Hamas gunmen who had tunnelled into Israel. The military said one of the infiltrators was killed and the rest of the group escaped back into Gaza.
About 32 tunnels, some as deep as 25 metres (82 feet), and dozens of access shafts have been uncovered since Israeli ground forces moved into Gaza following an air and naval barrage launched on July 8, the military said.
It said dozens of Palestinian gunmen have been killed or captured in clashes at tunnel entrances. In addition to the Israeli casualties in such incidents, a dog sent down to probe a one passage in southern Gaza never returned.
Israel sees the tunnel search as vital to fending off infiltrations in which its southern villages and army bases might be raided by gunmen.
Lieutenant-Colonel Roi, one of Tomer's battalion commanders, said several armoured vehicles - tanks, troop transports, D9 bulldozers and earth-movers, some with code names like "Bella" or "Cruella" - accompany each tunnel hunt, both for the heavy lifting and to provide a wall of steel around the foot-soldiers involved.
Hamas and its Palestinian guerrilla allies regard the tunnels as strategic assets against their militarily superior enemy. Each one of the secret passages costs $500,000 to $2 million to build, with each kilometre (0.6 mile) stretch taking about a year to complete, according to Israeli estimates.
Tomer's unit, the 188th Armoured Brigade, has found two tunnels and several access shafts so far. He said the searches were based initially on rough coordinates given following surveillance of suspected digs and other intelligence.
As the extent of the tunnel warren and the lethal threat it poses became known, accusations were leveled against the government and military, which warned about the phenomenon years ago but apparently did not prepare adequate counter-measures.
Yossi Langotsky, a geologist and retired army colonel who formerly advised the Defence Ministry, said his calls to develop a seismic technology to "hear" tunnel digs had been ignored.
A current Defence Ministry official, speaking to Reuters on condition of anonymity, said Israel was close to deploying a tunnel-detection technology. But were that to be used on the Israeli side of the border, it would still leave Palestinians free to carry out most of the dig under the frontier. Some Israeli officials have proposed that the military set up a long-term buffer zone inside Gaza. (Writing by Dan Williams; Editing by Jeffrey Heller, Will Waterman and Prudence Crowther)