Last night we learned the Barrick Gold Mine has been working prototypes . They weren't the only company testing the trucks, but nobody knew what exactly they were driving.

Some of the drivers at the mine gave their impressions on the F-150 for a Ford video. They say they knew they were driving a prototype, and instructed to "drive it like they stole it." Here's the clip:

The mining company, a Pennsylvania construction company, and a utility company had been sold F-150s in the current bodystyle with an aluminum cargo box. All they knew was that they were "driving something new."

Ford's Communications Manager Mike Levine jumped in to elaborate:

This is the first time Ford has deployed prototype vehicles to customers so far in advance of a product launch for extensive customer testing.

Six total units (two each) were delivered about 2.5 years ago to Barrick Gold, plus Walsh Construction in Pennsylvania and a regional utility company.

At Barrick, one F-150 has approximately 74,000 miles and the other approximately 95,000 miles. The photos were taken today at the Cortez Gold Mine, the largest operating gold mine in N. America, according to Barrick.

The customers didn't know they had trucks with aluminum cargo boxes. We only told them they were driving something new and we wanted to check in on the trucks from time to time. Barrick only found out about the aluminum cargo box after the 2015 was revealed at NAIAS.

By starting so early, we made changes to the production F-150, such as upgauging the cargo box floor and tailgate updates that improved durability.

Here's an aluminum-boxed F-150 in action on a hydroelectric dam site:

And here's another clandestine evaluation mule at the gold mine:

I've never heard of this kind of deep-cover for a new vehicle, but with a change as dramatic as an aluminum body to Ford's golden goose I can't say I'm too surprised they went to extreme lengths to do it right.

I hope we'll hear from the companies that have been using the trucks soon!

Images: Mike Levine/Ford