On Wednesday, explosive devices were sent to CNN headquarters at the Time Warner Center in Manhattan. Bombs were also sent to former President Barack Obama and former Secretary of State Hillary Clinton, in what New York City Mayor Bill de Blasio is calling “an act of terror.” This spate of would-be bombings have cast light on a little-known device used by police to haul away the explosive material called a “total containment vessel,” or TCV.

The TCV is designed to absorb the blast from a bomb. It’s been described as “an inside-out diving vessel.” In 2016, after a pressure-cooker bomb went off in Manhattan, police gave reporters a tour of the high-tech device.

“Instead of keeping the pressure out and keeping you alive in five fathoms of water, it keeps the pressure in,” [Lt. Mark Torre, the commanding officer of the NYPD’s bomb squad] explained. Should a bomb explode inside, tiny vents allow pressure to escape. “It sounds like a hammer hitting a piece of steel,” he said.

The NYPD has three TCVs located throughout the five boroughs to be deployed at a moment’s notice, counterterrorism Chief James Waters told the Daily News in 2016. Inside the chamber is a “basket” where the explosive rests to prevent explosion-triggering turbulence during the drive.

Such vessels are often capable of containing a blast of 25 pounds of TNT or more, and they are increasingly a common piece of equipment for police agencies across the country.

Images of the spherical TCV are now making the rounds on Twitter, with local news helicopters even tracking it as its hauled north to Rodman’s Neck, a peninsula in the Bronx where police destroy unexploded bombs.

The NYPD posted this video in 2016 highlighting its lineup of bomb detection and removal technology.