SCOTTSBLUFF, Neb. (AP) — The annual revival of the Pony Express has been chosen as the official event to celebrate Nebraska’s 150th birthday.

More than 700 horse-riders began the mail service trip in Missouri Monday and will travel nearly 2,000 miles via the Pony Express National Historic Trail to California by June 15. The 565-mile route in Nebraska is the longest trek among the eight states.

This year’s commemorative letter honors Nebraska’s sesquicentennial, describing the Great Platte River Road’s history and featuring the iconic Chimney Rock. The envelope will also showcase the U.S. Postal Service-issued Nebraska Sesquicentennial postage stamp.

Though the original Pony Express only operated for 19 months in the 1860s, it was the most direct means of connecting the new state of California with the rest of the U.S.