Cumbra Vieja

Yikes. There is a Wikipedia entry for this as well with a shorter synopsis.

The western flank of Cumbre Vieja volcano on the island of La Palma
in the Canaries is going to slide into the Atlantic one of these days: a
diagonal fracture has already separated it from the main body of the
volcano, and only friction still keeps it attached. “When it goes, it will
likely collapse in about 90 seconds,” said Professor Bill McGuire, director
of the Benfield Grieg Hazard Research Centre at University College London.
And when it goes, probably during an eruption, the splash will create a
mega-tsunami that races across the Atlantic and drowns the facing
coastlines.