Makes sense though. The complex Riemann-Zeta function (Zeta(x) = sum from 1 to infinity of 1/n^x) only converges for Re(x) > 1, else it is infinite. We can use analytic continuation - which I understand as extending the function such that it remains holomorphic (basically continuous but in the complex numbers and for some reason well-defined) and find the value of Zeta(-1) that way. The value of Zeta(-1) when viewed this way is -1/12. It's not really saying the sum of all natural numbers is equal to -1/12. It's really just a mathematical trick, just like: lim (x -> 0) x/x = 1 but this doesn't mean 0/0 = 1.