I will talk about the isomorphism types of torsion subgroups of elliptic curves over various
number fields, in particular, the growth of torsion when the base field is extended to the
compositum of all quadratic extensions.
Links:
[1] https://www.mpim-bonn.mpg.de/taxonomy/term/39
[2] https://www.mpim-bonn.mpg.de/node/3444
[3] https://www.mpim-bonn.mpg.de/node/246