Name a cash cow in this sprawling city of ragged slums and glass-walled office buildings and it's almost certain there's an organized crime syndicate behind it.
The illegal operations, routinely referred to as mafias, are everywhere. There's a land mafia that commandeers prime real estate, a sugar mafia that conspires to control sugar prices, and even a railway mafia that forges train tickets and pilfers locomotive parts.
For those on the city's bottom rung, however, the underworld entity they revile the most is the water tanker mafia, a network of trucking firms that teams up with corrupt bureaucrats to turn water into liquid gold worth tens of millions of dollars each year.
The water tanker mafia's prey can be found in slums like Karachi's Gulshan-Sikanderabad neighborhood, where every morning people buy water from the tankers, lug the plastic jugs back to their homes on wooden carts, then come back three or four more times in the afternoon and evening to buy more.
A family that makes $100 a month can spend as much as a quarter of that on water, which, elsewhere in Pakistan, costs pennies and flows out of household taps.
Water scarcity isn't the cause. Karachi has a steady water supply, and it has the network of pipes to pump ample water into every neighborhood, rich and poor.
But Karachi is also a city of opportunists forever on the prowl for under-the-table wealth. As municipal officials look the other way, businessmen illegally tap water mains, and use the makeshift hydrants to supply fleets of tankers that then sell water to businesses, factories and neighborhoods at inflated prices. As many as 272 million gallons a day are siphoned off by the trucks.
On a recent sunbaked afternoon, along a dirt lane filled with goats munching on piles of refuse, Momin Khan seethed as he filled another blue jug with water from a cistern replenished every other day by the water tankers.
"We're poor laborers -- we can't spare this much for water," said Khan, 27, a glass factory worker. "The water supply lines come right into this neighborhood, but there's never any water. So I buy the same water that I should be getting through the pipes for free. I've got no choice."
Karachi has nine hydrant locations where water supply companies can legally buy water and fill their tanker trucks. But scattered throughout the city are at least 160 illegal hydrants, said Ashraf Sagar, manager of the Orangi Pilot Project, a private organization that researches water issues in Karachi.
The siphoning takes place around the clock, Sagar said. It's done in the dead of night, but also in broad daylight.
Along Manghopir Road, a bustling Karachi avenue lined with grease-covered car repair stalls and appliance storefronts, it's easy to find a pair of tanker drivers standing on top of their trucks, filling up with a large blue hose from an illegal hydrant inside a red-brick building. Armed guards keep outsiders from meddling.
On average, a tanker fills up six times a day, Sagar said, siphoning as much as 41% of the city's daily water supply, an amount that generates $43 million annually for tanker owners, according to Orangi.
"With this much money involved, it's clear these are very wealthy people," Sagar said. "They're powerful mafias colluding with corrupt people in the government. So there's really nothing ordinary Pakistanis can do to stop it."
Shahnawaz Jadoon, a deputy administrative chief for the Gulshan-Sikanderabad neighborhood, said it was virtually impossible to clamp down on an enterprise that combines the clout of city government and the wealth of Karachi's powerful business circles.
At times, illegal hydrants are shut down by city officials, only to reopen a week later. Activists said they didn't know of anyone involved ever being arrested.
"The big reason why people don't get the water they're supposed to," said Jadoon, "is that if they did, this whole system, the tanker mafia and this corrupt network, would shut down."