Help please: What is this under the floorboards?

[UKMasters]

Hi, I'm a complete newbie when it comes to houses and DIY. I am currently looking to run some cables through from the first floor to the ground floor. It's inside a cupboard, so I don't really care about the look of it.

When I pulled up the floorboard I saw this underneath. The ridges appear to be made of 'cement'. I was wondering what they are and what their purpose is (if any)

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I presume this is part of the ceiling below, so I am planning to just drill through into the cupboard below - but I just wanted to check before I did anything!

Thanks for your help.

 

[Jackrae]

Looks like plaster which has been forced through the lath below and acts as a key as the ceiling was being plastered
It's perfectly normal and actually holds the ceiling from falling down so don't go breaking too much of it off.

 

[freddiemercurystwin]

Drill up from below, not from above, very gently! OR whole chunks will come down. On your head!

 

[UKMasters]

Thanks guys, really appreciate the quick response.

I'll put my hard hat on