Black Pit Bob-omb
From the Super Mario Wiki
In The Super Mario Bros. Super Show! episode "Rolling Down the River", Captain Koopa used a Black Pit Bob-omb to create a black pit in a river; Captain Koopa does this to help him stall his opponent, Mark Twang, from beating him in a race to become King of the River. They went to the Mouth of the River to ask how to continue and she told them that there won't be a pit if the river flowed around it. After that, Mario and Luigi built pipes to change the river's flow around the pit and continued the race.