I read the whole string of other comments and just have to make my own:
sometimes it's just better to do something right. Move the shed, get a
backhoe to pull it out and haul it off. Tank is probably 4' in diameter,
5' long, perhaps 400 pounds. Then it is done and gone, no further worries.

Next door neighbors had a 1000 gallon fuel oil tank buried underneath
thier 2 car garage floor. Even that took only about 4 hours to remove.

davefr wrote:
I buried a 500 gallon gas tank in the late '70's for farm use. I used
it for about 10 years and then pumped it dry.

Complete removal is impractical since a small shed is built on top of
it and I don't want to demolish the shed.

I'd really like to cut the top off and fill it with rock or dirt so
it's decommisioned once and for all.

I checked with the state DEQ office and they told me 500 gallon tanks
are unregulated. My only obligation is to report a spill but this
tank never leaked since I quit using it very early in it's life.

I'm not too fond of the idea of using a cutting torch. Even though
it's totally dry there could still be fumes and I don't want to get
dead. For the same reason, I really don't want to use an angle
grinder.

I'm thinking sawsall or pneumatic chisel.

Anyone know how underground tanks are decomisioned? I'd like to
remove a 5' X 2' portion of it's top.

Is there a safe way to make 100% sure the tank is vented such as
injecting an inert gas that's heavier then gasoline fumes to drive
them out?

Other suggestions?