Proof of work cryptocurrencies began with the promise of a more egalitarian future with a decentralized monetary system with no powerful entities in charge. While this vision is far from realized, these cryptocurrencies are still touted to be much more decentralized than traditional centralized systems. While it is well understood that cryptocurrencies are centralized, it is still unclear what the underlying causes are. This work aims to address this gap and examines some of the forces behind mining centralization. The internals of cryptocurrency mining is very opaque and difficult to study since it traditionally requires forming relationships with miners, who are typically reticent to share internal information about their competitive advantages. This work takes a different approach by combining large-scale statistical techniques with publicly available blockchain data in order to answer previously intractable questions. The crux of our analysis technique is based on the simple observation that some miners can utilize their hashpower more efficiently due to their position in the network. By teasing out that effect, we de-bias the mining power distribution to get a more accurate estimate. Using that de-biased mining power distribution, we can answer questions about the network position of miners in each cryptocurrency network. Finally, during the course of this study, we observed some unusual mining behaviors which we highlight.