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The Bitcoin TestNet is a network identical to Bitcoin in every respect except that the bitcoins on it are worthless because no one accepts them in payment or values them for anything other than testing purposes. In essence, TestNet btc are valuable because they are worthless, allowing for useful experimentation when developing.
I read somewhere that one way the TestNet can be prevented from acquiring significant value and mining power is by being reset from time to time so that TestNet btc remain easy to mine and acquire for testing purposes. This made me curious--has this ever happened before?
2Also note that the testnet3 commit mentioned by @Nick does not specify a new timestamp for the genesis block. Thus, most testnet blockchain explorers will display the testnet2 genesis date ('2011-02-02') for block '#0' (instead of the actual testnet3 genesis date, which is something '2012-04'ish). This can be a major source of confusion. Also, it appears that between blocks 513 and 514 a sudden temporal shift occurs as the block time jumps from '2011-02-03' to '2012-05-24' unexpectedly. – Priidu Neemre – 2015-04-07T13:36:18.163