That's a good point. Say you fill half the hole with containers of water. Let's assume it's 1km deep. Say we have a circular hole that's 2m across, that should

That's a good point. Say you fill half the hole with containers of water. Let's assume it's 1km deep. Say we have a circular hole that's 2m across, that should get you W = m * g * d = (3.14 * 1m^2 * 500m * 1000kg/m^3) * 9.8m/s^2 * 500m = 7.7 * 10^9 J or 7,700 MJ? That's a lot higher than the numbers being thrown around here. Edit: just noticed the comment about 500,000 kg -> 1.2MWh. Read that as MJ at first... d'oh.