It was the third time humans have dived to the deepest point in the ocean, known as Challenger Deep.
Victor Vescovo, a retired naval officer, said he made the unsettling discovery as he descended almost 10,928 metres (more than 10 kilometres) to a point in the Pacific Ocean's Mariana Trench - the deepest place on Earth.
Mr Vescovo found a plastic bag and lolly wrappers, as well as four new species of prawn.
The team said its scientists were going to perform tests on the creatures found to determine the percentage of plastics found in them. At 26,247 feet (8,000 m), they observed Mariana snailfish and supergiant amphipods (Alicella species) - creatures about 20 times larger than typical amphipods.More news: China Responds to Trump's New Tariffs, Targeting $60 Billion in US Exports
They also collected samples of brightly-coloured rocky outcrops from the seabed.
"It is nearly indescribable how excited all of us are about achieving what we just did", Mr Vescovo told BBC News.
The latest descent, which reached 10,927m beneath the waves, is now the deepest by 11m - making Victor Vescovo the new record holder. "This submarine and its mother ship, along with its extraordinarily talented expedition team, took marine technology to a ridiculously higher new level by diving - rapidly and repeatedly - into the deepest, harshest, area of the ocean". The dive was 52 feet deeper than any previous manned dive, Vescovo's Five Deeps Expedition said. Atlantic Productions for Discovery Channel/Handout via REUTERS.
A type of long-appendaged amphipod was discovered at the bottom of the Challenger Deep, in the Mariana Trench.More news: Google Spotted Testing Automatic Car Crash Detection in Android Q
After the conclusion of the dives, the submersible - built to withstand 1000 bars of pressure - will be given to researchers at science institutions to continue exploring the ocean's depths. In 2012, movie director James Cameron made a solo trip into the Mariana Trench, however Mr Vescovo's team broke Cameron's previous records by about 11 metres.
This is a widespread problem for sea creatures, even those living extremely deep in the ocean, a recent study has found. Vescovo, a private equity investor, is funding the expeditions.
The deepest deep-sea dive ever may have just found them, the BBC reports. The group is using a submersible called Limiting Factor to complete its challenge.
They also observed a variety of critters. As well as the Mariana Trench, it's now completed surveys of the Atlantic Ocean's Puerto Rico Trench, the South Atlantic's South Sandwich Trench and the Java Trench in the Indian Ocean.More news: Schumacher's family open bidding war for life story
The final challenge will be to reach the bottom of the Molloy Deep in the Arctic Ocean, which is now scheduled for August 2019. The pressure at the bottom of the ocean is equal to about 50 jumbo jets piled on top of a person, according to BBC News.