A project known as Biosphere 2 attempted to sort out the basic science of oxygen/carbon transactions in the global ecosystem by setting up a 'closed' ecology. In form it was like a gigantic futuristic garden centre, with plants, insects, birds, mammals, and people living inside it. The idea was to keep the ecology working by choosing a design in which everything was recycled.
The project quickly ran into trouble: in order to keep it running, it was necessary to keep adding oxygen. The investigators therefore assumed that somehow oxygen was being lost. This turned out to be true, in a way, but for nowhere near as literal a reason.
The 'missing' oxygen wasn't escaping from the building: it was being turned into carbon dioxide. So why didn't they see increased carbon dioxide levels? Because, unknown to the experimental ecologists, carbon dioxide was being absorbed by the building's newly set concrete as it 'cured'.
Crazy stuff. Did you know that it is possible to take a guided tour of Biosphere 2? It's only an 800 mile drive from my house!