ASIC

Basics

• "An application-specific integrated circuit, is an integrated circuit (IC) customized for a particular use, rather than intended for general-purpose use. Application-specific Integrated Circuit, an ASIC is the current ‘top of the line’ technology used in Bitcoin mining.  It is able to generate orders of magnitude more hashes per watt of power than the next best technology."
• Bitcoin mining is a competitive endeavour. An "arms race" has been observed through the various hashing technologies that have been used to mine bitcoins: basic CPUs, high-end GPUs common in many gaming computers, FPGAs and ASICs all have been used, each reducing the profitability of the less-specialized technology.

ASICs going “dark"

• "ASICs in this context are specialized machines used to mine one or more cryptocurrency chains. When they go dark (the hash power drops and they stop being used), they are left idle and you no longer see their hashrate show up on the blockchain; this can be due to a number of reasons, the most usual of them being that the owners can no longer find electricity cheap enough for them to mine profitably."

Sunk Cost

• "Sunk cost is the large amount of money they prepaid to buy ASICs. That money is recovered by mining the coin over the coming months /years, as the rig cannot be used for anything else; so doing things that affect such recovery (lowering coin price by attacking transactions on the chain is one of them) is unlikely to be considered."