Mercury is already significantly denser than water, with a density of approximately 13.5 times that of water. However, if you were hypothetically considering a scenario where a substance could be made 20.5 times heavier than water, it would imply creating a material with an extraordinarily high density.
Increasing the density of a substance to such an extent would require significant changes at the atomic and molecular level. One way to achieve this could be through the creation of a new material or compound with a unique atomic arrangement, potentially involving heavy elements or dense packing structures. However, it is important to note that such a hypothetical material would likely require a level of scientific understanding and technological capability far beyond what is currently available.
It's worth mentioning that density alone doesn't determine the strength or usefulness of a material. Other factors like mechanical properties, stability, reactivity, and availability are equally important when considering the viability and potential applications of a new material.
The hypothetical scenario you mentioned of a substance being 20.5 times heavier than water is quite challenging to envision in the context of our current scientific knowledge and capabilities. Creating such a material would require groundbreaking scientific discoveries and advancements in materials science and chemistry.