Modals for speculating about the past

Atlantis may have sunk in the ocean.

• sure

• not sure

UFOs might have been new airplanes.

• not sure

• sure

Bigfoot could have been a big gorilla.

• sure

• not sure

People must have done the crop circles.

• sure

• not sure

Aliens __ the Nazca lines. I'm totally sure!

• may have done

• must have done

The Incas __ Machu Picchu. I don't know.

• might have built

• must have built

I'm pretty sure that the Pyramids of Giza __ by old egyptians.

• must have been built

• could have been built

Probably the Jarjachas __ big llamas.

• may have been

• must have been

They are convinced the Triangle of Bermudas __ a portal to other dimension.

• must have been

• might have been

Maybe the boogeyman __ just a children's myth.

• could have been

• must have been