Thread by @ScarbsTech

Porpoising. The cars can be seen to bounce up and down at speed.
This isn't new and not only related to big underfloor tunnels.
The car gets its downforce from the wings and underfloor. The ride at low speeds gives no problems.
#F1

As speed increases the downforce increases too. This compresses the tyres and suspension.
As the underfloor gets closer to the ground, it works even better creating even more downforce
(this is ground effect).
Ride height continues to reduce with the aero load

This reaches a critical stage, where the ride height is too low and the airflow in the underfloor stalls.
This suddenly reduces downforce, the reduction in load uncompresses the tyre/suspension, lifting the car back up to a higher ride height

This sets off a cycle, called porpoising.
As normal ride height returns, the airflow reattaches and downforce is created again, again compressing the suspension.
The call will bounce until its speed changes.
Not every car will porpoise, some designs are more prone to it.

The solutions are:
Less stall prone underfloor shapes, the double kick diffuser was supposed to be a solution
Slots in the floor edge to keep airflow attached at low RH, but this can hurt max downforce
Heave element suspension set ups to control the car at low ride heights