Steam
The water from a tsunami can create a lot of energy when made into steam. Steam power used to be all the rage back in the day. Steam comes from water, water comes from tsunamis, so logically, tsunamis are a great source of energy.
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In order to contain this steam energy and store it for later, you lock it up in a box with a key (き). That way the energy can be used at a later time. Provided you don’t lose the key, that is.