90 GB have mysteriously disappeared from the hard drive

[Algorithm.]

90 GB of space has mystically dissapeared from the SSD.
I have four folders on my SSD which total 148,33 GB
(3,74 + 5,57 + 131,39 + 7,63 = 148,33 GB)

However, disk information shows that 238,22 GB are used.
That’s a difference of 89,89 GB.
Where has 89,89 GB disappeared?

External image source:
http://oi57.tinypic.com/2qcmbev.jpg

 

[Algorithm.]

Hidden files?

Could it be hidden files?
If so - how can I see them?

 

[macnjack]

Disk Inventory X is a decent tool for seeing where your loss of space went.

Have you emptied the trash?

 

[glenthompson]

Possibly Time Machine snapshots. Do a search for how to turn those off.