i remember when i be a kid saw my first horror movie in the late 70 and this one scar me to death . it full of atmosphere and like if you know john carpenter thing and but there no monster or alien which kill one after another and the horror be more like real life . the loneliness of the place where these scientist be work with the ape be more than claustrophobic and the more you stand by the cast and go through that story the more mad you will get . the score give it part perfectly to it and old 70 synthesizer sound bring mad and scary atmosphere . i really hope this will find it way to dvd . a real underrated classic .