These tools are very advanced for their time. Ancient people __________ (know) more than we thought.
must have known
This metal object was found inside rock from millions of years ago. It __________ (be) modern.
can’t be
These footprints in the snow __________ (belong) to a large animal—or maybe Bigfoot?
might belong
A plane disappeared without a trace. It __________ (crash) in the ocean.
might have crashed
The statues on Easter Island were transported across the island. How?
They must have had a way to slide or walk them.
No one has seen the Loch Ness Monster clearly. It __________ (be) a log or wave.
could be
People keep spotting UFOs in different parts of the world. What could they be?
They might be drones or military aircraft.
This strange circle in the field __________ (make) by someone as a prank.
might have been made
The cave paintings are very old and detailed. They __________ (be) important to ancient people.
must have been
The pyramids were built thousands of years ago with perfect symmetry. What do you think?
They must have used clever tools or systems.
Some planes and ships disappear in the Bermuda Triangle. What do you think happens there?
There might be a natural explanation like strong currents or weather.
We saw a bright light move across the sky very fast. It __________ (be) a plane.
could have been