There is a big difference between the east and west sides of OI. I've read about this somewhere, but I'm lazy so I'll just quote AI which matches what I recall:
- Eastern Side (Money Pit Area): This area features a drumlin with soil conditions characterized by limestone and anhydrite (a calcium sulfate mineral). The geology here includes "Lawrencetown" till, a mix of roughly 50% sand, 30% silt, and 20% clay. The bedrock is composed of Mississippian Windsor Group limestone and gypsum. This area is prone to water infiltration, forming cavities below sea level.
- Western Side (Western Drumlin): The western drumlin has different, harder characteristics. It features deeper zones that were glacially compressed into schist and slate. The side-wall soil in this area contains higher amounts of clay.
With that much sand & silt and so little clay, there is no way you could dig an un-cribbed 100-ft deep shaft on the east side with any expectation of survival. The west side is another story, so at a minimum it would be possible. But there is zero evidence anyone ever did that, so there is plenty of deniable plausibility. A low frequency GPR scan would quickly put this to rest.