I hold a PADI rescue diver certification and have made several dives to 95+ feet. I have never used trimix. 5 ATM is 4 ATM of water pressure and 1 ATM of air. You add 1 ATM for every 33 feet so 4 ATM of water is 132 feet - the limit for recreational diving. If you want to go deeper or you want to stay down there for more than a few minutes you have to get in to technical diving and then you can learn to use trimix. Anyone with an advanced certification and a deep specialty can get to 5 ATM of pressure.

Also, 1 ATM is 14.7 PSI. So 5 ATM is 73.5 PSI. Still enough to crush you and to keep you from breathing through an unpressurized hose, but not hundreds of pounds per square inch.

NO. RHEL 6 wasn't released in the 90's, but Red Hat 6 was. Red Hat has changed names and re-started their version numbers.