  We report experimental evidence for a crossover between a liquid-like state
and a gas-like state in fluid methane (CH4). This crossover is observed in all
of our experiments, up to 397 K temperature; 2.1 times the critical temperature
of methane. The crossover has been characterized with both Raman spectroscopy
and X-ray diffraction in a number of separate experiments, and confirmed to be
reversible. We associate this crossover with the Frenkel line - a recently
hypothesized crossover in dynamic properties of fluids extending to arbitrarily
high pressure and temperature, dividing the phase diagram into separate regions
where the fluid possesses liquid-like and gas-like properties. On the
liquid-like side the Raman-active vibration increases in frequency linearly as
pressure is increased, as expected due to the repulsive interaction between
adjacent molecules. On the gas-like side this competes with the attractive Van
der Waals potential leading the vibration frequency to decrease as pressure is
increased.
