With everything but the hammer my thought is an earthquake may have pushed them much deeper into the Earth's crust.
I am sure if it was a post-industrial earthquake there would be a historical record of such an event. If the earthquake was pre-industrial, then we still need an explanation for the artefacts.
There is also evidence that many of these artefacts were found securely embedded in their host coal or rock formations; see below. Thus it would seem, unless all of these claims are hoaxes, the artefacts date to a period prior to the coal or rock formations.
Ancient artefacts embedded in coal or rock formations