Work has officially begun on "Crisis on Infinite Earths," this year's crossover between The CW's DCTV shows.Former Arrow showrunner Marc Guggenheim, who now serves as an executive consultant for both the original Arrowverse series and DC's Legends of Tomorrow, made the announcement with a photo tease. The picture centers on a big card with the "Crisis" logo, which is glows with an ominous red light.RELATED: Crisis on Infinite Earths Will Be Arrowverse's Longest Crossover Yet"Working on a little somethin somethin," he teased in the caption.

Guggenheim wasn't specific about what kind of work he is performing for the five-show crossover. However, considering that the Arrow writers room reconvened just last week, his Tweet probably refers to the planning and writing process, as it is far too early for filming to start on the crossover.

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Seeing as every Arrowverse show closed their latest seasons with a "Crisis" tease, this fall's slate of shows will likely build to the unprecedented event. "Crisis" will be split into five parts, with each Arrowverse show providing a piece of the story: Arrow, The Flash, Supergirl, Legends of Tomorrow and Batwoman. Spanning a whopping five hours, the crossover will air across two quarters, which indicates it will be split over the winter hiatus.

Slated to air in Fall 2019 on The CW, Crisis on Infinite Earths currently has no announced air date.