in the 1980s, empty C-5 airlifters had a big problem, it took 15 tons of rock to solve it, the guys loading those twin 15 ton cement clocks called them pet rocks

To stay within contract funding limits, the Air Force cut the weight and structure support for the aircraft wings. This resulted in a profound shift of the center of gravity aft

The weakened wings “flapped” at cruise, the wing tips moving up and down in flight. The two primary effects of this blunder to save money required re-winging all the aircraft with the original, stronger wings, and demanded pet rocks for local flights, secured far forward in the cargo bay

Pet rocks remedied this problem until the new wings were installed.

https://theaviationgeekclub.com/this-rock-was-placed-in-the-forward-part-of-an-empty-c-5-cargo-bay-to-avoid-the-shifting-of-freds-cg-to-its-aft-limit/