Abstract: A balloon signalling system which includes a hollow flexible casing having a container section with an upper open end and a bottom closed end, and a cap for placement on the upper end of the container section. A hollow flexible flask having an opening is disposed in the container section. Disposed in the flask is a reactable material which when contacted by a certain solution, produces hydrogen gas. Also disposed in the flask is a cartridge which is breakable when bent, with the cartridge containing the certain solution. A string or other tether is held in the container section of the casing, with one end of the string being attached to the casing. Also included is an inflatable balloon dimensioned to fit in the cap when deflated and having a mouth through which gas may pass into and out of the balloon. Positioned in the mouth is a one-way valve for allowing hydrogen gas to enter the balloon and for preventing discharge of gas therefrom.