There have been many interesting presentations at the Mitchell Theatre over the years.
There was the Tom Keneally Anniversary Lecture and a talk on the building of the Harbour Bridge, and last month Peter FitzSimons spoke about his latest book, Eureka.
But the lecture scheduled at the Pitt Street venue next month is in a different realm.
Aliens. Tall, white ones, at that.
Charles Hall, a nuclear physicist, ex-military American weather specialist and author is touring Australia with his claims that the United States military has been in contact with an alien species for years.
He says the aliens would dress as humans and visit Las Vegas.
''In 1964, when I was a weather observer at Nellis Air Force Base in Nevada, I witnessed interactions between the military and a group of mysterious tall, white, human-like extra-terrestrials,'' the Vietnam veteran said from his home in Albuquerque.
''Their craft are capable of travelling faster than the speed of light because Einstein was wrong about relativity.''
Mr Hall said no cameras were allowed at the site, which has since been wiped off the map.
When he arrived, a colleague had wanted to lock them in the weather station and not go to the other end of the building ''because that's where he had encountered them. There was a group of five, two men and three women, and they had come to go to the bathroom.
''When you encounter the 'tall whites' it's such a shock, you are not sure if you are looking at a ghost or an angel, or if you are dreaming,'' he said.
Mr Hall said he had contact with three types of aliens - the ''tall whites'' (about 2.5 metres tall), ''greys'' (with yellow/orange skin) and the ''Norwegians with 24 teeth'', who look like humans and speak English.
They were principally involved in technology transfers with the US military, and tall whites would dress like humans and go to shows at Caesar's Palace.
Mr Hall said he originally penned his experiences into a fictional book to disguise the identities of those involved - but what was written was true. He said US officials had kept it all quiet because people weren't ''emotionally ready'' to accept alien life forms.
Mariana Flynn, the president of UFO Research Centre NSW, believed Mr Hall had encountered aliens. ''The topic is a red rag to a bull to some people, particularly those in authority,'' she said.
The lecture and book signing is on April 6.