Police in Leeds say they are "very concerned" for a suspected burglar who escaped police by climbing through a storm drain and into the sewer.
Officers arrived at commercial premises on Mabgate, just outside the city centre, at around 11.30pm on Sunday, after receiving a call about a break-in.
Upon arrival, they saw two people leaving through the roof. It is thought one of them then entered the sewerage system through a nearby storm drain.
West Yorkshire Police have consulted specialists who have expressed concerns about the dangerous conditions in that part of the sewer system. They said they called experts from Yorkshire Water and West Yorkshire Fire and Rescue Service who carried out a "detailed systematic search."
No-one has been found.
Superintendent Pat Twiggs, from City & Holbeck Division, said: "We can find no indication of that person having got out of the sewer safely and we are therefore very concerned for them due to the extremely hostile environment in the sewer.
"We are making wide-ranging inquiries to try to identify the individual involved but we would like to hear from anyone who has concerns that someone they know is missing and could be involved in this incident.
"We would also like to hear from anyone who has any information which could help us trace and locate anyone who may have been involved.
"I would like to reassure them that our primary focus is on ensuring the safety of those involved in this incident and the investigation into the burglary is completely secondary at this stage.
"I would hate to think that someone could be sitting on information which could help us to resolve this situation regardless of where they feel their loyalties lie."
Anyone with information can call police on 101 or Crimestoppers anonymously on 0800 555 111.
It was not clear whether the other suspected burglar remained at large.