  For general gravitational collapse, inside the black-hole region,
singularities $(r=0)$ may arise. In this article, we aim to answer how strong
these singularities could be. We analyse the behaviours of various geometric
quantities. In particular, we show that in the most singular scenario, the
Kretschmann scalar obeys polynomial blow-up upper bounds $O(1/r^N)$. This
improves previously best-known double-exponential upper bounds
$O\big(\exp\exp(1/r)\big)$. Our result is sharp in the sense that there are
known examples showing that no sub-polynomial upper bound could hold. Finally
we do a case study on perturbations of the Schwarzschild solution.
