Learn French: Il y a des os anciens dans cette tombe. - There are ancient bones in this tomb.

You are right that il y a followed by a time expression means "ago" (like il y a trois jours for "three days ago"). However, its original and most common meaning is "there is" or "there are" when it is followed by a noun, like des os (bones) in this sentence.

Why is anciens placed after the noun? Doesn't it mean "former"?

The meaning of ancien changes depending on its position! When placed before the noun, it means "former" (like un ancien professeur for "a former teacher"). When placed after the noun, as in des os anciens, it literally means "ancient" or "very old".

How do you pronounce des os? Is the "s" silent?

Yes, in the plural form des os, the end of the word is completely silent and it sounds just like the letter "O" (and you link the "s" from des to it). However, be careful with the singular form un os (a bone): for the singular, you actually do pronounce the "s" sound at the end!

Why do we use cette instead of ce for "this"?

Tombe is a feminine noun in French. Therefore, you must use the feminine word for "this", which is cette. You would use ce for a masculine noun, or cet if a masculine noun starts with a vowel.