When a steam pipe beneath Lexington Avenue exploded on Wednesday, it sent skyward a brutal rain of hot water, concrete, and mud, and also served as a reminder that beneath the city is a vast, unpredictable infrastructure that sometimes fails.

“You look into a New York City street and you’ll be amazed,” the president of the Energy Resources Group, Leonard Shapiro, said. “It’s like cutting into the human arm.” There are 105 miles of metal pipe carrying steam that leaves plants at 350 degrees Fahrenheit, as well as thousands of miles of water and natural gas piping, electrical wiring, and subway tunnels. Some sections of the grid, like the steam pipe that exploded, are almost as old as the Titanic and other parts are as newly installed as last week. “We are an older city with infrastructure that was sophisticated in its time,” the author of “The Works: Anatomy of a City,” Kate Ascher, said. “In any one of those systems, there is older pipe and newer pipe.”

Each of the entities handling the city’s infrastructure — from Consolidated Edison to the Department of Environmental Protection — is constantly repairing pipes and wiring, but as newspaper reports from over the years show, there is no early warning system good enough or pipe strong enough to stop man-made disasters.

Since a steam pipe explosion in Gramercy Park in August 1989 that left three people dead, there have been at least 17 steam pipe explosions, according to published reports. New York has also seen in that time a citywide blackout and building collapses.

In fiscal 2007 there were 581 water main breaks, a spokesman for the Department of Environmental Protection, Michael Saucier, said. The department monitors each foot of water main at least every 36 months, and in some high-usage areas even more often. Sewers are also regularly inspected.

“It’s a hostile environment underneath the streets of New York,” a professor of infrastructure at New York University’s Wagner School of Public Service, Rae Zimmerman, said. “It’s a very dense, crowded area with leaking water, people digging under the streets, and vibration.”

In addition to Consolidated Edison’s monitoring programs for electrical wires and steam pipes, the company has contracted Honeybee Robotics to develop a robot that can move through steam pipes like an inchworm, repairing cracks and welding stronger seams.

The robot, called W.I.S.O.R., has to stand high heat conditions and be able to adjust to pipes that are between 16 and 24 inches in diameter, the chief engineer of the project, Roopnarine, said.

A filmmaker who made a documentary about the robot, Michael Negroponte, said Charles Emery of the New York Steam Company started laying down steam pipes beside inventor Thomas Edison, who was simultaneously burying electrical cable, in the late 1800s. “It was a visionary plan in the 19th century,” he said, adding that most skyscrapers of the 1930s were economically feasible because they were entirely heated by steam. “Unfortunately, it does erupt sometimes.”