Technicians prepare a gas-sealing test above an underground cavern at a hydrogen energy project inside Gasunie's HyStock site. The underground cavern, formerly used to store natural gas, will now be used to house hydrogen. Each cavern is around 300 - 400 metres high and 50 - 80 metres wide, big enough to contain the Eiffel Tower. Since a hydrogen molecule is much smaller than that of natural gas, engineers have to apply additional sealing to prevent leakage. The technicians are inserting a pipe to pump nitrogen, which is more inert than hydrogen, into the cavern for testing.