Some people have noticed that archaeologists often stop digging when they get down to a level that shows artefacts consistent with the commonly believed theories of how civilization advanced and when technologies migrated from place to place.
In North America it was accepted that the first humans came 10,000 tears ago so when they got that deep, they stopped. But then a river embankment in Yukon, Canada collapsed and human artefacts were found in a 30,000 year old layer. Some Americans went back to old sites, dug below 10,000yo and found artefacts.
So they may simply have found more archaelogy while digging, and artefacts that clearly came from Europe or Asia.
Alternatively, some believe that oil is MANUFACTURED in the earth's crust. If you dig deeper wells than the ones that petroleum companies normally dig, you can tap this oil. Some say that the Russians are already tapping this supply and used it to build up their emergency funds back when oil was pricier than today, in the early years of Putin's rule before the price dropped in half. Supposedly you can detect this new oil because it has different ratios of the carbon isotopes than oil made from dead plants and animals.
Perhaps this is what he means, because it would mean we have hundreds more years before oil runs out.
Could be other minerals too. What if there is a layer of diamonds at a certain depth and they are actually almost as common as sand?
Or a layer of gold and platinum which is occasionally pierced and melted by volcanoes and distributed to the top layers of the crust.
Or what if there are more serious upwelling currents in the deep crust than we realized, and Yellowstone is not the only supervolcano under the USA but there is also one under Cleveland and another under New England and they ARE RISING!?