The explanation is that the antarctic tectonic plate wasn't where it is now. Plus we've been in an ice age for like 2.5 million years. So when the plate that currently covers the south pole was closer to the equator and the temperature of the earth was substantially warmer at the same time, then it had a rainforest.

Antarctica has been at the (current) south pole for >100 million years

I thought it was nearer 90M years. Which matches up with the sample pretty good.

True polar wander sounds like one of the most terrifying natural disasters I could imagine.

Gradual polar shift happens over millions of years; specifically characteristic rate of true polar wander is 1° or less per million years.

If the shifting occurs as a “Cataclysmic” polar shift [1] or geomagnetic reversal [2], the periodic reversal of the Earth's magnetic field (effectively switching the north and south magnetic poles) — I agree that it would likely have terrifying results for the inhabitants of Earth.

[1] https://en.m.wikipedia.org/wiki/Cataclysmic_pole_shift_hypot...

[2] https://en.m.wikipedia.org/wiki/Geomagnetic_reversal

I mean, it's a very slow motion disaster -- so slow it can't be a disaster for us.