Many of the world's historic domes have been around a thousand years, a testament to their design and construction. Here are three European domes that would make a great destination for senior travelers:

Pantheon: Rome, Italy. Built in 126 AD, it remains the world's largest non-reinforced concrete dome. It sits in the heart of Rome and is a spectacular site to visit. Inside, the space is cathedral-like. The distance to the top of the roof opening (called the "oculus") is the same as the diameter of the circular floor, both 142 feet.

There's no admission fee at the Pantheon and it's open from 9 a.m. to sunset on weekdays and until 1 p.m. on weekends.

Duomo: Florence, Italy. The formal name is the Cathedral of St. Maria del Fiore, built in 1296, and the dome is a wonder of Renaissance engineering. Italian architect Brunelleschi used the Pantheon as an inspiration, but created many new building and engineering techniques in this building. He invented machines to hoist the giant stones and bricks to the heights necessary to complete construction.

The $15 ticket grants access to the Duomo, the dome and cathedral areas including the crypt and museum.The Duomo is open from 8:30 a.m. to 7 p.m. on weekdays and until 5:40 p.m. on Saturdays. It is closed to visitors on Sundays because of church services.

St. Paul's Cathedral - London, UK: Visitors can climb 259 steps to the circular interior Whispering Gallery", giving spectacular views of the church, which was built in 1675. The acoustics allow someone whispering to be heard over 100 feet across the dome on the other side.

Climb another 271 steps to venture outside to the "Golden Gallery", which encircles the outside of the dome. This is also one of London's best vantage points for view the city. St. Paul's is open from Monday through Saturday from 8 a.m. to 4 p.m. and is open only for worship on Sundays. Adult admission is $30, kids are $15. Admission includes guided tours and multimedia guides.