What is/would be on the other side of the planet if it is flat?

If the Earth is flat what is/would be on the other side?

Would it be lava, like what is believed to be at the center of the Earth if it is a sphere? If so, is this why people say the temperature increases when going deeper underground?

Is this same thing as how we don't know what is in the center of the Earth if it is sphere?

If the Earth is flat what is/would be on the other side?

Would it be lava, like what is believed to be at the center of the Earth if it is a sphere? If so, is this why people say the temperature increases when going deeper underground?

Is this same thing as how we don't know what is in the center of the Earth if it is sphere?

You are right, there is no way to know what is under the Earth below 8 miles in our day. Any conversation concerning the other side of our Earth would be pure speculation.

Well, speculation is the nature of all of flat earth theory, as it stays mostly in the realm of guesswork due to the difficulty, scale, and cost of experiments to prove any matter of the theory.

Who knows?
There are predictions that may be made depending on model. Some might have rock heated by whatever accelerates the Earth (in a model with UA), some posit the Earth is at the edge of space, and so there is no 'other side'...
It seems rather pointless to speculate. This is, after all, a fairly irrelevant matter.

Some believe it to be just rocks. Others believe that the earth rests on the back of four elephants and a turtle.

Some believe it to be just rocks. Others believe that the earth rests on the back of four elephants and a turtle.

It's obvious what is on the other side, more of our spherical planet. I should know, I've been on both sides.