Gas-giant planets on greater than 10 au orbits are the current exoplanet population probed via ALMA. There is now indirect evidence in both dust and CO gas observations for multiple gas-giant planets embedded in disks (e.g., HD 163296). The two proposed mechanisms for giant planet formation are core accretion and gravitational instability. The latter is an attractive solution to explain some of the detected exoplanet systems and the growing evidence for the "fast" (< 1 Myr) formation of gas-giant planets in disks. However, this mechanism relies on the disk having a high mass - higher than gas-mass estimates from C18O observations are suggesting. As the gas is expected to carry 99% of the disk mass in the early stages of disk evolution a robust measurement of this is necessary. In this poster, I present ALMA observations of the rarest carbon monoxide isotopologue, 13C17O, in the HL Tau disk. This species provides a more robust determination of the gas mass and reveals that the C18O emission is optically thick and therefore of the gas mass of the HL Tau disk has been significantly underestimated. I then show what this new mass means for the gravitational stability of the system.