a synthetic fluid that is liquid at standard outdoor temperatures, contains oxygen (and thus is breathable if jumped into) and has the right density, viscosity, and other characteristics to be able to make an average human survive jumping into it from 10 meters
and considering said synthetic liquid would cost a bunch of money to make (for the equipment to produce a suitable amount of it) it also checks the "valuable" requirement, and after you're done with the experiment you keep it and you can simply sell it for a high per-liter price to get lots of money
