Got a question about this the other day; here’s what I know – we’ll add some more information later:

1. An exploding manhole cover is  a predictor of nearby manhole cover explosions. But not necessarily adjacent holes. So if there’s a problem with one – great caution about all area manholes is indicated. 

2. They’re very heavy – and they’ve been measured going as high as 400 feet vertical. After that, they descend (32fps squared, less  drag). Do the math.

Here’s one resource

“How Exploding Manholes Work ,”  by Kevin Bonsor, on HowStuffWorks.com