This is the fourth episode in a series of five videos from the Seamount Corals Survey 2018, a month long voyage on CSIRO's Investigator to map and survey deep-sea coral reefs across an area of underwater seamounts south of Tasmania. The voyage criss-crossed many seamounts in and near the Huon and Tasman Fracture marine parks, which are home to both pristine and previously fished coral reefs. These two parks are part of a larger network of Australian Marine Parks that surround Australia’s coastline and protect our offshore marine environment. A curious feature of one of the southern seamounts is that it hosts the world’s only known aggregation of deep-water eels. We have sampled these eels twice before and were keen to learn more about this rare phenomenon. Using an electric big-game fishing rig, two egg-laden female eels were landed from a depth of 1,100 metres: a possible first for the record books. The voyage also gave us the opportunity to run a unique experiment to calibrate our echosounder recordings. An echosounder is a device designed to send a pulse of sound down to the seabed below. This pulse of sound then reflects back when it encounters the seafloor, or life, such as fish, squids, jellies and krill. Near the seamounts, approximately 1000 m below the surface, we are observing schools of fish that we are eager to discover more about, specifically their numbers. Orange roughy are of special interest as they have been known to occur here in “feeding aggregations” over the summer period. However, as orange roughy have oil-filled bladders rather than a gas bladder, they have a weak reflection and are difficult to count. The experiment involved lowering a very large gas-filled glass ball off the side of Investigator, deep below the water’s surface. Using sophisticated positioning systems and ship manoeuvrability we have been able to measure the glass ball position and strength up to depths of 1200 m. We are optimistic that this will improve our ability to count fish at longer ranges!