S
supersparks
yes, it cuts steel 
but your point being?
ok so as thermo says, you give it a few wraps arround the tank, then you detonate, which, will have one of two effects
1) it will cut the steel, about 5mm all round, which will not be totaly continuus due to being thicker in placesa which will hold it together, so the clean cut formed will then compress down and seal the gap, leaving a wall of fire with the fuel trickle(and thats what it is likly to be) alight - easy to control by fire service....
2), it will cut the the tank in half leaving you with a few million gallons of fuel running out, will cause mass damage but not explode on the scale seen
your defeating your own logic here because were back to the "why dosnt fire run down the gas pipe" science.....
oh and as for putting the cord arround the whole tank, dont forget it has two skins that are seperated by a cavity so would be useless....
the only way it could explode like that is to have a fuel/air mix, hell you even stated it yourself!
so at the mo it looks most likly to be as dabaldie suggests with an empty tank but prior to filling with water /non flamable liquid to disapate the mixture, maybe a float switch or something that should have been deactivated in the tank wasnt shut off and ignited - who knows
the rumbling before the explosion, hmm...... well, what makes a rumble? rushing gas, can be caused by fire rapidly burning, prehaps a large vent duct or something was full of fuel air mix and ignited at one end and burnt through very rapidly, candle effect, then reached a tank it was connected to, went inside, the volume of gas was burning faster than it could excape and then something has to give- bang
but all we can do is speculate with educated guesses, but one thing i would point out is that, its unlikly to be the explosive cutting cord because the witnesses would have seen the flash and bang before it did its job.
but your point being?
ok so as thermo says, you give it a few wraps arround the tank, then you detonate, which, will have one of two effects
1) it will cut the steel, about 5mm all round, which will not be totaly continuus due to being thicker in placesa which will hold it together, so the clean cut formed will then compress down and seal the gap, leaving a wall of fire with the fuel trickle(and thats what it is likly to be) alight - easy to control by fire service....
2), it will cut the the tank in half leaving you with a few million gallons of fuel running out, will cause mass damage but not explode on the scale seen
your defeating your own logic here because were back to the "why dosnt fire run down the gas pipe" science.....
oh and as for putting the cord arround the whole tank, dont forget it has two skins that are seperated by a cavity so would be useless....
the only way it could explode like that is to have a fuel/air mix, hell you even stated it yourself!
so at the mo it looks most likly to be as dabaldie suggests with an empty tank but prior to filling with water /non flamable liquid to disapate the mixture, maybe a float switch or something that should have been deactivated in the tank wasnt shut off and ignited - who knows
the rumbling before the explosion, hmm...... well, what makes a rumble? rushing gas, can be caused by fire rapidly burning, prehaps a large vent duct or something was full of fuel air mix and ignited at one end and burnt through very rapidly, candle effect, then reached a tank it was connected to, went inside, the volume of gas was burning faster than it could excape and then something has to give- bang
but all we can do is speculate with educated guesses, but one thing i would point out is that, its unlikly to be the explosive cutting cord because the witnesses would have seen the flash and bang before it did its job.
