What Is the Risk of a Single Mining Pool Accumulating a 51% Hash Rate?

If a single mining pool controls 51% or more of the network's total hash rate, it gains the theoretical ability to execute a '51% attack.' This attack allows them to potentially double-spend coins, prevent new transactions from gaining confirmation, and reverse transactions. This fundamentally undermines the security and immutability of the blockchain, leading to a catastrophic loss of confidence and value.