Three employees of the Mombasa County government have died while unclogging a manhole at the Makupa roundabout.
An incident report said four others escaped death with injuries after they succeeded in moving out of the drainage system, leaving the three men stuck inside the main drainage systems.
Four exhauster trucks are on site pumping storm water out of the tunnel with assistance from the Kenya police.
Mombasa Governor Abdulswamad Nassir who spoke at scene said the three drowned while trying to clear the debris from the smaller drainage but unfortunately there was an influx of water.
"We were able to get four people out and alive but there are still three people that we are hoping we will be able to remove them, however, there is one more person who is currently down there. There is no other way to get there except a person has to go down," he said.
The four who were rescued were rushed to hospital where they were treated and are currently out of danger.
The governor said the tunnel all the way up to the Indian Ocean adding that "chances that they are on the other side is pretty slim."
Abdulswamad said the clogging of the tunnel was as a result of an engineering blunder by the Kenya National Highways Authority (KeNHA).
"The engineers came and looked at what was the cause of the problem and they found out that when KeNHA was doing the road, they basically built a drainage system on top of another existing drainage system," he said.
He said the drainage is narrow and the ongoing rains worsened the situation.
