Nah that’s a common misconception, a lot of the ocean is mapped to enough resolution to know there’s basically nothing there. A lot of the ocean floor is a barren desert with nothing much going on. Some parts have been thoroughly studied, where interesting stuff is going on and where we’ve put infra like all of the undersea cables (which are a lot). All of it is mapped to some resolution and some parts at higher resolution (you can see on Google maps lines where ships have gone through with better mapping abilities). So saying it hasn’t been “explored” or “mapped” is simply false, it has been mapped and has shown to be not interesting. So no nobody went down there, nobody got pictures or centimeter level mapping, but we know how water and sand looks so why bother? It’s this implication of something unknown and mysterious, whilst in reality we’ve just been efficient in what to find. And yes there are new species found in the ocean every day, but you’d need to be an expert to even tell them apart from other species we already know about. There isn’t something big and interesting we don’t know about. It’s a nice story to tell, but it isn’t real.