[superdude12]

Considering it doesn’t have to carry much weight, why couldn’t they make it out of wood to cut costs?

[mrj]

Dirt is pretty heavy. As the article said, the next step is to add native plants to the top.

[mulmen]

I would be shocked if material costs are within an order of magnitude of the labor and design costs.

[quickthrowman]

There are (at least) thousands of tons of soil on top of the reinforced concrete.