Abandoned Building
Noun
はいきょ
昨日廃墟で鬼ごっこをしたので、筋肉痛になりました。
I have muscle pain because I played tag in the abandoned building yesterday.
Obsolete ruins are places that are abandoned because nobody can run them anymore (due to being obsolete, or due to not having money, or due to whatever else). These things are ruins or abandoned buildings. People like to explore these, a lot.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.