i guess the voltage would be rather low... but current capacity might be enough to melt your drill bit, if you decide to drill hole through both of the electrodes :-D
Ok, TIL that the high capacity of a supercap doesn't translate into a ginormous voltage (Wikipedia: "A supercapacitor [...] is a high-capacity capacitor, with a capacitance value much higher than other capacitors but with lower voltage limits"). Good to know...
Specifically in this case they were limited to less than 1.23V because that's when you start to get water breakdown in the electrolyte solution (if I've read it right).
Current kills. If your foundation cracks, say, in an earthquake, the electrodes can become exposed. Flooding is also common in earthquakes. As another poster said, floor is lightning.