An underwater microphone has picked up two dolphins seemingly carrying on a conversation as if they were two people, the Telegraph reports. When researchers in Russia recorded a pair of Black Sea Bottlenose dolphins in a pool, it sounded as if they were chatting—and quite politely, at that.

Each dolphin would listen to the other without interruption before responding. While researchers have known for a while that dolphins could communicate via clicks or whistles, this study found that they also altered the frequency and volume of their sounds.

As a result, researchers say the aquatic duo could form sentences of up to five "words," though scientists had no idea what they were going on about.

More from Newser:

The dolphins' communication skills exhibit "all the design features present in the human spoken language," writes lead researcher Vyacheslav Ryabov in the journal Mathematics and Physics.

"Their language can be ostensibly considered a highly developed spoken language." Now, says Ryabov, it is up to humans to decode dolphin speak. Their high-frequency sounds are beyond the human range of hearing, notes the Christian Science Monitor, but scientists could use equipment to capture and mimic those sounds, then respond in a way the dolphins would theoretically understand.

Australian researchers have previously pinpointed particular whistles the mammals use to communicate ideas such as, "There's food over here," per the Telegraph. (Last year, scientists using 3D imaging figured out how dolphins "see" with sound.)

This article originally appeared on Newser: Scientists Catch Dolphins Chatting