A white utility truck. A generator humming nearby. A cordon of orange cones. An open manhole, steam dancing like campfire smoke. It’s a familiar tableau, repeated day and night on Boston streets. If you’re like me, you’ve wondered: What is in those manholes, anyway? And what’s with the steam? It turns out there’s a labyrinth of steam pipes underneath the city — 17 miles worth, actually. A company called Veolia Energy uses the pipe network to pump steam from power plants to office buildings, hospitals, and other facilities — 70 percent of Boston’s high-rises, in fact — which convert it to heat and hot water. When it rains, water gets in the manholes, and with the steam pipes so hot, the water turns to vapor. Workers siphon it out in part to keep hot steam from building up dangerously underground. On a recent morning, a couple of guys from Veolia were emptying a manhole on Atlantic Avenue. The generator powering the pump whined as it worked, and the water made a trickle as it dropped from a yellow hose into a storm drain.