A search was launched in Chorley following reports of a man waist deep in water, say police.
Officers received reports of a man in Anglezarke Reservoir just off Moor Lane at around 10.15pm on June 8.
Police searched the area using a police helicopter but were unable to locate the man.
A spokesman for Lancashire Police said: "We received reports of a man waist deep in water. A search of the area was made but the man was not located.
"Our enquiries are ongoing."
Anyone with information is asked to contact police quoting log reference of 1542 of June 8.