Ordinary people are usually cremated these days in Korea. After cremating, some people put the ash of bones in an urn and keep an urn at 납골당. Other people scatter the ash of bones in the sea or in the mountain. There are 납골당( a building which is prepared for keeping urns) for the bereaved. But still some people( who have their own mountains or prepare for land for buying a body) bury a body in the ground. You said you are looking for your mother's grave. I guess your mother's bereaved might know the exact place... People can't say where your mom was buried by the reason that your mother lived in Seoul. Because a person who lived in Seoul but a body could be buried in other province. Long time ago, a married woman was buried at husband's 선산( ancestor's mountain). But everyone couldn't prepare 선산 so some people buried a body in other's mountain secretly. Even though people got buried a body in the ground and made a tomb, if a tomb wasn't cared by family for a long time...a tomb couldn't be found easily. Because most of tombs were located in the mountains, if a tomb wasn't cared for a long time then a tomb was coved with weeds and people lost the place of tomb.

Hi Mary, is it safe to assume you were adopted at some point in your life and are now looking for your deceased mother? If so, I would start by getting her full name (in Korean) and her date of birth, if you can, then contact the nearest Korean consulate or embassy for assistance in locating her burial site or related information. What MunKyoung said about many being cremated instead of getting buried these days is very true. I wish you the best.