Bulkor

Meet Bulkor, a steel ball who gets squeezed from every side at once and barely notices.

When pressure rises all the way around an object, it shrinks a little in volume. The bulk modulus says how hard that is. It is the extra pressure divided by the fractional change in volume, and the minus sign is there because more pressure means less volume.

Water has B ≈ 2.2 × 10⁹ Pa. At 4000 m down in the ocean the extra pressure is about 4.0 × 10⁷ Pa, so water there is squeezed by only about 1.8 percent. Steel has B ≈ 1.6 × 10¹¹ Pa, so the same pressure shrinks Bulkor by just 0.025 percent.

Gases are far easier to squash, which is why air bubbles shrink so much as divers go deeper.

The bulk modulus also sets the speed of sound in a material. Stiffer materials carry sound faster, which is why sound travels about four times faster in water than in air.