Geological evidence suggests that during a future
eruption, Cumbre Vieja Volcano on the Island of La Palma
may experience a catastrophic failure of its west flank, dropping
150 to 500 km3 of rock into the sea.
...
Lateral collapses of oceanic island volcanoes rank amongst
the most spectacular natural events on Earth. Although no
such lateral collapse punctuates the historical past, residual
debris found on the seafloor evidence their abundance in recent
geological time. Moore (1964) first identified the remains
of lateral collapses off the flanks of Hawaii. Since then,
dozens have been recognized adjacent to island volcanoes in
nearly every ocean (Moore et al. 1994; Keating and McGuire,
2000). These observations constrain not only the geography
and frequency of lateral collapses, but also their magnitude
(up to 5000 km3 of material), extent (to 300 km length) and
ferocity (underwater speeds to 140 m/s).

Originally posted by Janus
There was an incident at a place called Lituya bay in Alaska. A landslide resulted in a wave 520 meters high. Imagine that but a 20 times bigger and more powerful.

The rockslide occurred along the eastern wall of the Gilbert Inlet (see figure above). The mass of rock striking the surface of the bay created a giant splash, which sent water surging to a height of 1720 feet (see figure above) across the point opposite the inlet...
Eyewitness accounts from the few unfortunate boaters who happened to be anchored in the bay for night, state that the wave was at least 100 feet tall at its maximum height near the head of the bay.