Every so often I see one of these odd, squarish trailers being pulled down an interstate and wonder, "What are those things?" Last fall, while driving down U.S. 23 near Toledo, I spotted a pair parked in a lot and swung around to check 'em out.

Two owner-operators pulling for Powersource Transportation in Griffith, Ind., had these in tow, and were taking a break. The said the strange trailers were water storage tanks used on construction sites, and they were hauling these from Dover, Del., to Denver, Colo.

At a work site a tank sits on the ground with its rear wheels in the air. The long bottom provides plenty of support for up to 20,000 gallons of water (at 8 pounds per, that'd be 160,000 pounds). When contractors no longer need it, the tank is emptied so it's ready to move. A crane lifts the nose so a truck-tractor can back under and couple to its kingpin. The tank has road-legal equipment, including a 22,000-pound axle with 22.5-inch wheels and tires, air brakes, lights and mud flaps, so it becomes a semitrailer.

Wade Services in Ellisville, Miss., makes the "frac" tanks (so-called because many are used to supply fracturing fluids at oil and gas wells). I put in a call to a sales manager at the company but didn't get a call-back.

However, the company's website described the tanks. Standard models are listed as 21,000-gallon capacity, are made of 3/16-inch steel, and have manhole hatches and other hardware up top, OSHA-approved ladders, and the trailer gear. Wade now makes doubles versions so two can be hauled at a time, the website said.

Wade's specifications didn't include length and weight, but each tank appeared to be about 35 feet long and the o-o's figured a tank's empty weight at about 20,000 pounds. They said Powersource moves all kinds of trailers that aren't cargo vehicles - circus wagons, for instance - and these tanks are among them. So, now I know.