The Excavator monument is complete and is now in-game!
This isn't just a static prop, it is a fully functioning mining excavator powered by Diesel. You can find the required fuel at the Junkyard, Oil rigs and at the top of the Dome.
You can also exchange low grade fuel for diesel, but it isn't cheap.
To use the excavator, you'll need to clear out the scientists guarding it and make your way to the engine room. Once you've added the fuel, you can power up the engine and make your way along the catwalk to the control room and select a resource type to mine. The mined resources will be deposited in equal amounts at two drop points. This means you'll need at least 4 people to operate it without a chance of someone stealing either the fuel or the output.
Also take note that RF #4777 will be activated as long as the engine is on, so watch out for counter raids.
You can find a more detailed write-up from the guys at Rustafied.com