A United Boeing 737-900, registration N75436 performing flight UA-401 from Reno,NV to Denver,CO (USA), landed on Denver's runway 16R at 15:55L (21:55Z) but struck its tail onto the runway surface. The aircraft continued landing, rolled out without further incident and taxied to the apron. A passenger reported the aircraft slammed into the runway twice, the pilots didn't make any announcements though. A post flight inspection detected damage to the tail, the tail strike was then reported to tower. The occurrence aircraft is still on the ground in Denver about 24 hours after landing.