REUTERS - A network of Palestinian tunnels running under the frontier town of Rafah is now water-logged, destroyed by Cairo to sever what it says is a weapons smuggling route out of Gaza for Islamist insurgents in Egypt's Sinai desert.

Egypt's pumping of salt water from the nearby Mediterranean into the tunnels is not only creating a mess as it rises to the surface. Palestinian officials say it is also contaminating water supplies as well as threatening to wreck farmland and spread disease.

Open gallery view General view of the exterior of a smuggling tunnel that was flooded by Egyptian security forces. November 2, 2015. Credit: Reuters

Local residents say that at the peak of the tunnel business, after Hamas Islamists seized the Gaza Strip in 2007 and Israel tightened a closure of its crossings into the enclave, nearly 2,500 underground passages snaked under the border with Egypt.

Open gallery view A Palestinian worker is lowered on a rope into a smuggling tunnel, November 2, 2015. Credit: Reuters

The direction of traffic was mainly into Gaza. Commercial goods - and weapons smuggled in separate tunnels controlled by Hamas and other militant factions - flowed in defiance of what Palestinians and many of their supporters decried as neighboring Israel's siege.

Open gallery view A Palestinian worker is lowered into a now-flooded smuggling tunnel on the border between Egypt and the southern Gaza Strip. November 2, 2015. Credit: Reuters

In 2008-10, some tunnel owners were said to have become dollar millionaires as they shifted everything from Hummer vehicles and washing machines to cows and sheep through the underground system. Hamas imposed a tax on shipments.

Open gallery view A Palestinian worker repairs a smuggling tunnel into Gaza after it was flooded by Egyptian security forces. November 2, 2015. Credit: Reuters

At one point an estimated 22,000 Palestinians worked in the tunnel "industry." However, it shrank markedly in 2010 after Israel, under international pressure to ease restrictions on commercial imports into Gaza, allowed more goods.

Open gallery view Palestinian workers rest as they repair a smuggling tunnel, November 2, 2015. Credit: Reuters

Then this September, battling an insurgency in northern Sinai, Egypt decided to shut down the tunnels once and for all. Determined to halt what it said was an arms flow in the opposite direction, from Gaza to the militants, it cleared the area on its side of the border and began pumping water into the underground maze, collapsing the land.

Open gallery view A Palestinian worker walks inside a smuggling tunnel, November 2, 2015. Credit: Reuters

Tunnel-builders said Egypt has pumped in water several times since September, and that over the course of a few weeks had done more damage to the network, which once accounted for an estimated 30 percent of Gaza's imports, than Israeli bombing had caused over the past two decades.

Open gallery view A Palestinian worker sits inside a smuggling tunnel, November 2, 2015. Credit: Reuters

Now, the diggers said, fewer than 20 tunnels remain for commercial goods, with easy-to-smuggle cigarettes the main contraband. No one can, or will, say how many weapons tunnels remain - a secret that is guarded by Hamas and other armed groups, which last fought a war with Israel in 2014.

Open gallery view A Palestinian worker is lowered on a rope into a smuggling tunnel, November 2, 2015. Credit: Reuters

What is left is an environmental mess, residents and local officials said, with the sea water polluting underground drinking reserves. Egypt's campaign, said Tamer al-Sleibi, water department director in the Palestinian Environment Quality Authority in Gaza, could weaken the foundations of homes already on shaky ground due to tunnel-building and make land unfit for agriculture in areas near the frontier. There is also a health risk as the water turns stagnant, allowing mosquitoes and other disease carriers to breed.