The article mentions a key to make it persistent - let them decompose under water, where oxygen supply is insufficient. Flooded open-pit mines are a good canida

The article mentions a key to make it persistent - let them decompose under water, where oxygen supply is insufficient. Flooded open-pit mines are a good canidate. I imagine that turning them into dead zones would create bogs and sequester much carbon. Mines for brown coal are likely in a suitable geographical location since they used to be bogs before (this is how brown coal came to be).