How does the Tsunami develop?
Everybody knows that what goes up must come down. This is particularly true for water which always likes to form a nice flat surface. So once the mountain of water has risen up the next step is for the sea to level itself out.
The mountain of water comes back down. This pushes the water that was underneath it outwards. The force of the water moves through the ocean causing an underwater force that travels for hundreds of Kilometres. The force of the water can reach speeds of up to 800kmh as it surges through the ocean. The energy is underwater and is not noticeable on the surface.
As this force travels through the ocean it may eventually reach the shore. At this point, the sea becomes shallower. However, the energy in the water is still the same. The enegery is compressed and the water is pushed upwards. This is how the energy is transferred from being underater into waves on the surface.