Combination of pressure in the tank ( about 30 PSI) and a suction cup. You can pressurize and then "bump" the perimeter with the heel of your palm, and sometimes it will pop right up. You would want to bump in the more "open" area across the to versus the outer crown where the tank rolls of to the left side. I'd try the forward, then aft sides. If it won't pop with pressure and a bump, I'd go to the suction cup. It appears to be a large "can" type dent, so you have a good chance of it working. As large as it is, it's not likely the metal is stretched, just "sprung" backward.