Mike Caldwell is known in the Bitcoin community for creating Casascius physical bitcoins, one of the earliest and most popular forms of physical Bitcoin tokens. These collectible items, introduced in 2011, contained a small piece of paper with a Bitcoin address and a private key, covered by a tamper-evident hologram. Caldwell's creation played a significant role in the early days of Bitcoin, providing a tangible representation of the digital currency and contributing to its popularization. However, he ceased production in late 2013 due to regulatory concerns.