Water blocks look like a solid aquarium.
(10 cm)³ = 1 kg
(1m)³ = 1000 kg = 1 t
Air for example is about 1.1 to 1.3 kg per m³, depending on temperature, helium about 1/7 and hydrogen about half of helium.
Perhaps Carbon-Buckyballs (nano-footballs containing for example vacuum) could have a...