Bitcoin blockchain technology is a distributed ledger of nodes authorizing transactions between anonymous parties. Its key actors are miners using computational power to solve mathematical problems for validating transactions. By sharing blockchain's characteristics, mining is a decentralized, transparent and unregulated practice, less explored in HCI, so we know little about miners' motivations and experiences, and how these may impact on different dimensions of trust. This paper reports on interviews with 20 bitcoin miners about their practices and trust challenges. Findings contribute to HCI theories by extending the exploration of blockchain's characteristics relevant to trust with the competitiveness dimension underpinning the social organization of mining. We discuss the risks of collaborative mining due to centralization and dishonest administrators, and conclude with design implications highlighting the need for tools monitoring the distribution of rewards in collaborative mining, tools tracking data centers' authorization and reputation, and tools supporting the development of decentralized pools.