Answer: Underground facilities that previously contained fossil fuels
Explanation:
In the attached passage, the author speaks on how natural carbon sinks such as trees and oceans can not be trusted with holding Carbon dioxide indefinitely because oceans are reaching their holding limits and trees are being cut down on a massive scale.
The author suggestion to mitigate this is to convert underground facilities such as oil and gas wells that previously held fossil fuels until they were pumped out. The author claims that the technology to do so is already present and that there are ways this could be made inexpensive.
