Boreholes drilled into the frozen surface of the polar regions (to retrieve ice cores for research) can be up to 3.5km deep.
Dropping a chunk of ice into one of these echoey vertical tunnels makes for very pleasing sounds indeed (best heard with the sound up or headphones on).
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They should mess with people by throwing down some alien-type stuff and then filling it back in before the next borehole is drilled
This should definitely win the Friday competition!
The Droppler Effect…