  The water molecule is a key ingredient in the formation of planetary systems,
with the water snowline being a favourable location for the growth of massive
planetary cores. Here we present Atacama Large Millimeter/ submillimeter Array
data of the ringed protoplanetary disk orbiting the young star HL Tauri that
show centrally peaked, bright emission arising from three distinct transitions
of the main water isotopologue. The spatially and spectrally resolved water
content probes gas in a thermal range down to the water sublimation
temperature. Our analysis implies a stringent lower limit of 3.7 Earth oceans
of water vapour available within the inner 17 astronomical units of the system.
We show that our observations are limited to probing the water content in the
atmosphere of the disk, due to the high dust column density and absorption, and
indicate that the main water isotopologue is the best tracer to spatially
resolve water vapour in protoplanetary disks.
