This is one of two underground passes that I know of in Kuching. It runs underneath Jalan Tun Razak (near to Foochow Road/Jalan Tun Razak traffic light junction). It only allows pedestrians and motorbikes to pass.
The other under-pass runs between Dewan Tun Abdul Razak building (new museum) and Padang Merdeka. Of course the one downtown doesn’t look the least bit like a monsoon drain outlet 😐