http://ttevhjjsjxz6uqqcjkbig5cycd7n7xv7cmd6f5fcvrqaaa7f3bj36wad.onion/technology/security/pow/index.html
But this defense does not solve all DoS problems in the Tor Network. We can imagine scenarios such of an Onion Service from a news agency that
suddenly publishes a very popular news article, attracting more legitimate
users than it used to. A huge flock of legitimate users, all willing to solve
client puzzles, could deny the service functioning but without being considered
as an "attack".