Giant underwater avalanche decimated Atlantic seafloor 60k years ago

Fluidized bed movement. In the reef hobby we use filtration that operates on this principle - in essence, instability with fluid movement of semi uniform particles allows those particles to move as if they were a liquid if that makes sense.

The base layer acts like a set of bearings mixed with lubricant, allowing the avalanche to roll a lot further than seems possible.

EDIT: liquefaction can have very strange effects.

Take the mass of a few cubic miles of rock, let it fall (slide down an underwater slope) a mile-ish, and there's an ample supply of 0.5mv^2.

And, whether above or below water, really huge avalanches have dynamics that seem quite counterintuitive. (Due to the square/cube law and other effects.)

Maybe via:

https://en.wikipedia.org/wiki/Turbidity_current

https://en.wikipedia.org/wiki/Submarine_canyon

If you look at google maps on ocean floor near any major river you'll see it continue for hundreds of km. Longest is 2400km in bay of bengal.

Oh, yes. Depends on the avalanche's size (& location & etc.) - but (say) this one is hypothesized to have killed ~1/4 of the at-the-time population of Britain:

https://en.wikipedia.org/wiki/Storegga_Slide

Yes. Close to us, there was a landslide-caused tsunami in Nice, France back in 1979: https://en.wikipedia.org/wiki/1979_Nice_tsunami

I think there was one 100 years ago somewhere on the Canada coast that caused a tsunami and killed some people.

Can also happen in inland lakes: https://en.wikipedia.org/wiki/Tauredunum_event

See: the Forth Estuary.