Mad Max

There are a total of 31 minefields in the game: two in each main region and then one in Deep Friah's territory. They are considered part of the threat of each region, but they cannot be marked from Vantage Outpost balloons like every other type of threat. They are only marked when you drive near/on them. You'll likely notice and drive on many of them - they are open areas with lots of blown up cars spaced out around the area. They're marked on your map by a red target symbol.

In theory, the way to find and clear all the minefields is to first complete the 'Dinki-Di' Wasteland mission when you first get to Jeet's stronghold. This unlocks the Dog and Buggy for use. With it unlocked, you can drive it out of any stronghold. When you get somewhat near a minefield, the dog will stand up and bark and point in the direction of the minefield. This, however, is a massive pain to aimlessly drive around trying to find minefields. Instead, firstly, wait until you've cleared all other threat in the region before attempting this (mainly because the Buggy has no weapons or nitro boost, so you're not really equipped to fight off enemies while looking) because you'll likely have stumbled across the majority of them inadvertently while doing everything else. For any regions where you're missing minefields, refer to THESE IMAGES over on Steam that show each region with all threat marked.

Once you've arrived at the minefield, the dog will stand up and bark and point in the direction of a mine. Drive in the direction the dog points and when you get near a mine it'll be marked. Hop out and hold as you approach it to defuse it. Get back in the Buggy and repeat. Every minefield has three mines to defuse.

Optionally, you could also just drive your normal Magnum Opus over the minefield. If you run over a mine, it'll blow up your car immediately (giving you the five seconds to jump out) but it will still count as defusing the mine. In practice, this is a really inefficient way to take care of the mines though. The minefields are big enough and the mines are small enough that you'll end up wasting time trying to find the mines you can't see to try to run over them. The Dog and Buggy are far more efficient.