A routine training flight by the U.S. Navy’s E-6B Mercury “doomsday plane” captured the attention of sky-watchers this week in Colorado.

The four-engine command-and-control plane based on Boeing Co.’s 707 airliner on Wednesday took off from Travis Air Force Base in California and circled over Denver before continuing on to Tinker Air Force Base in Oklahoma.

It was apparently the hour-long racetrack holding pattern that turned the heads.

“Did you see this today?” a newscaster at the KMGH, the local ABC affiliate, asked during a broadcast. “There was this plane just circling the metro, circling and circling, and many of you called us asking what was going on.”

The station initially reported the identity of the plane “a mystery” but provided an update the following day.

In a telephone interview with Military.com, Lt. Leslie Hubbell, a spokeswoman for Naval Air Forces in San Diego, said there was nothing unusual about the plane or the flight. The Federal Aviation Administration directed the Mercury to fly the pattern it did around Denver, she said, presumably because of air traffic.

All 16 E-6s in the Navy’s inventory are B models after upgrades were completed to A variants more than a decade ago, according to the Navy League. The planes are estimated to cost roughly $150 million apiece.

More recently, the service began another modernization program to extend the life of the aircraft from 27,000 flight hours to 45,000 flight hours, according to the organization. The work includes adding new antennae, data links and radar terminals.

In case you were wondering where the moniker “doomsday plane” comes from, the planes serve as airborne command posts for strategic forces and relay communications for the service’s fleet of 14 Ohio-class ballistic missile submarines. Each sub can carry up to two dozen Trident D5 nuclear missiles.

Two squadrons, the “Ironmen” of VQ-3 and the “Shadows” of VQ-4, employ more than 20 aircrews from Tinker Air Force Base, Oklahoma, to fly the planes and keep them operational, according to the Military.com’s Equipment Guide.