Live in a old house built in the 1940s.
I rent been here 6 years.
As time as gone on we've noticed on the landing & small bedroom there's gaps being dented into the carpet from the floorboards, they are really creaky.
Today it feels like a hole slowly starting in the carpet/floorboards! 
I've always rented new built properties so never had this issue before.
Whose responsible us or the landlord please? And would they actually fix it? Becoming worried one of us will fall through at some point! We also don't have any house inspections so it's never been picked up.