It's known fact. He survives well on Carrier as well, that's not a showing. There was stated that the underground complex was city-sized, just like Carrier.
Though...Jack can fight at a desent level at least for a short while in the area close enough to the big settlements but still outside of them (I've posted the scan from Authority\Planetary crossover in the old Authority RT but it wasn't taken there).
Especially taking into account that it's neither urban environment nor pseudo-urban place like Carrier or city-sized underground base...which means that Jack is weakened in those scans.
Also one of my favourite things (Jack powering up by the energy he gets from the city and then ripping Regis apart) wasn't included by Digi Though he posted a scan with pwning a speedster twice and hasn't corrected it yet
Though it's stated that being in complete wilderness (like in the middle of Amazon jungle or Sahara desert with not even a small settlement nearby) kills him in 15 minutes...to be more exact, 15 minutes is a deadline after which the damage caused to Jack's body is almost incurable.
I know about the Carrier but I still felt it was worth mentioning. I was focusing on the jungle part of what Bendix said and somehow overlooked the "underground complex" part. Jack being able to treat a jungle like a city (if it was big enough) would be impressive, and that's what I was thinking.
Marvel has made a lame animeish ripoff of him recently. Veda of the Order team- a teenage girl with an oversized hammer. Though her urbanopathic abilities are weaker than Jack's and she has no superhuman physical treats, she doesn't have Jack's major weakness and is able to stay outside urban environments as long as she wishes.
Put Jack in the middle of Amazon rainforest or Siberian taiga and he'll be comatose after 15 minutes.
Godhead (a telepathic villain who feeds on energy of his believers) establishes his mindcomtrol over Jack. Jack resists it, but under villain's control he reshapes Los Angeles into Transcendence city and does some other stuff. (please log in to view the image)(please log in to view the image)
Some feats of Swift. She drags the already mentioned fat lady (who weights probably more than 400 pounds) straight through the wall of building and shoves a huge piece of debris into her mouth. (please log in to view the image)(please log in to view the image)
That happens after Swift and Angie free Jack from Godhead prison and Angie injects Jack with some neurostimulants to help him fight off Godhead's mindcontrol completely. (please log in to view the image)
Jack phases into ground to change clothes (Godhead kept him naked for some reason) and then with casual fingerclick crushes a skyscraper down on one of the Godhead's minions. (please log in to view the image)(please log in to view the image)