Past - More sure
The books could have been read by many people.
Past - Less sure
The pirate might have found the treasure.
Past - More sure
There must have been a dreadful disaster which happened to the Mayan.
Past - Less sure
The Atlantis might not have been existed on Earth.
Past - Less sure
The ship might have been destroyed by a terrible storm.
Present - Less sure
The pizza could be made by a skillful chef.
Present - More sure
The elephant must be in the park.
Past - Less sure
The mouse might not have been fed.