One of the most powerful investors in the world is standing firm in his belief that shopping malls are heading for a collapse in the next three years.

Carl Icahn, whom Forbes ranks as the 61st-richest person in the world, is now the single biggest holder of credit-default swaps on the CMBX 6 index, The Wall Street Journal reports. CMBX 6 tracks 25 commercial mortgage-backed securities, and its performance largely corresponds to U.S. malls overall.

Icahn made his name as a hedge fund manager, but his forays into the shorting position on malls have been funded by his personal fortune and only began in the past few months, WSJ reports. If CMBX 6 drops, it would reflect an overall pessimism from the market in mall owners' ability to pay back their loans — and could bring in a return of $400M for Icahn based on his current position, according to WSJ.

The 10-year CMBS loans associated with CMBX 6 were largely taken out in 2012, in the early growth years after the Great Recession, but before the effects of the e-commerce revolution came into focus. Many have wondered about (and bet against) the ability of borrowers to pay back or refinance those loans when they mature in 2022.

So far, short-sellers have been burned by the mall sector for the past couple of years as landlords have shown the ability to backfill enough vacated retail space to either keep lease payments coming or negotiate refinancing deals. Credit-default swap holders lose money when an index rises in value, and CMBX 6 has risen steadily in the past 12 months.

Some short-sellers have given up on trying to wait on malls to die. Ethan Yip's hedge fund, Alder Hill Management, shut down its entire operation earlier this year due to the losses sustained by shorting CMBX 6, WSJ reports.

Other hedge funds, like heavyweights Putnam Investments and AllianceBernstein Holding, have taken bullish positions opposite Icahn. But the multibillionaire is prepared to wait out short-term losses he has already begun incurring and may commit even further to the CMBX 6 credit-default swaps ahead of the loans coming due in 2022, WSJ reports.