Russia’s throttling of natural gas to Europe is endangering the continent’s race to sock away enough fuel to ride out next winter.

In energy circles, Europe’s summer months are known as the “filling season.” Lower demand for natural gas allows the bloc to replenish its vast underground storage network for colder months, when consumption soars. Storage includes salt and porous-rock caverns, aquifers and depleted gas fields. This year’s effort to top up these reservoirs has become a closely watched gauge of Europe’s energy security.

This story originally Appeared on Yahoo