We present our work on visual analytics tools to support the analysis of Bitcoin mining pool evolution. Mining blocks are a critical component of the Bitcoin ecosystem, helping to keep the system secure, valid, and stable. At the same time, mining is a resource-intensive activity that continues to get more and more difficult. Mining pools have emerged to address this issue and to ensure a more stable and predictable income by sharing computing power. Yet, increased centralization of the mining power is also not without dangers (e. g., the 51% attack), and, thus, it is important to better understand and analyze mining pool activities in Bitcoin. Here, we report three contributions: our extensive data collection on Bitcoin mining pools, our development of two custom visualizations, and our first exploratory data analysis leading to hypotheses and documented activities about pools' main features such as market share, reward rules, or location.