While recent advances in reasoning models have demonstrated cognitive behaviors through reinforcement learning, existing approaches struggle to invoke deep reasoning capabilities in multi-turn agents with long-horizon interactions. We propose DeepMiner, a framework that elicits such abilities in deep search agents by introducing a dynamic context window and challenging training tasks. We design a dynamic context management strategy that utilizes a sliding window to selectively drop distant tool responses while preserving all assistant reasoning traces, eliminating the dependency on external summarization models. To provide training tasks that genuinely demand long-horizon reasoning, DeepMiner further employs a reverse construction method to generate complex yet verifiable question-answer pairs from authentic web sources, ensuring both difficulty and reliability. Through reinforcement learning on Qwen3-32B, we develop DeepMiner-32B, which achieves substantial performance improvements across multiple search agent benchmarks. DeepMiner attains 33.5% accuracy on BrowseComp-en, surpassing the much larger DeepSeek-V3.1-671B, and demonstrates consistent improvements on BrowseComp-zh, XBench-DS, and GAIA. Notably, our dynamic context management enables sustained interactions of nearly 100 turns within commonly used 32k context length, effectively addressing the context limitations that constrain existing multi-turn agents.