In China, 2015 has been the year of the missing corporate executive. Again and again, without warning, some of the country's richest and most powerful business titans have vanished.

Some eventually resurfaced and returned to their posts; others did not. Their absences are seldom explained.

Most of the disappearances are part of an anti-corruption crackdown waged by President Xi Jinping since he took office in 2013.

Experts had expected the campaign to wind down, but this summer's massive market crash has led to a new wave of disappearances. Regulators are cracking down on alleged insider trading and "rumor-mongering."

Here are the stories of five executives, some of whom are still missing.

1. Guo Guangchang, Chairman of Fosun

Guo Guangchang is the man behind Fosun Group, which deals in everything from real estate to entertainment. Guo's fortune is estimated at roughly $8 billion, and he often describes himself as a student of Warren Buffett.

He went missing in early December. Two of his Hong Kong-listed companies -- Fosun International and Fosun Pharmaceutical -- were quickly suspended from trading.

Fosun would later say that Guo was "assisting in certain investigations carried out by mainland judiciary authorities." When shares started trading again, they plummeted.

Photos have since surfaced on social media showing Guo attending company events in China and New York. He is also rumored to have recently visited Canada. But his absence has never been fully explained.

2. Yim Fung, CEO of Guotai Junan International

Brokerage Guotai Junan reported a few weeks ago that it had been unable to reach its CEO, Yim Fung, since November 18.

Local media reports suggested Yim had been detained as part of an investigation into former Guotai Junan executive Yao Gang, who served as vice chairman of China's market regulatory agency.

For a month, neither the company -- nor the Chinese government -- provided clarity.

On Wednesday, the company announced that "Dr. Yim, in his personal capacity, had been assisting in certain investigations carried out by Mainland authorities during the period he was unable to be reached."

Guotai Junan also said that neither Yim nor the company were the subject of the investigations, and that he would resume his duties at the company immediately.

3. Mao Xiaofeng, President of China Minsheng Bank

Early this year, bank president Mao Xiaofeng was detained by China's top anti-corruption watchdog, the Central Commission for Discipline Inspection, to "assist with investigations."

Minsheng Bank filed a stock exchange announcement on January 31 saying that Mao had abruptly resigned via letter for "personal reasons." Months later, there's still no clarity or confirmation on what exactly happened to Mao.

Minsheng shares have plummeted 24% this year.

4. Zhang Yun, President of Agricultural Bank of China

Zhang Yun, the president of Agricultural Bank of China, is the most high-profile bank executive to be detained. Last month, state media reported that he was assisting officials with an investigation.

Agricultural Bank of China is one of the country's four massive, state-owned banks. It has assets worth $2.7 trillion, making it the world's third-largest bank, according to SNL Financial.

In December, Zhang resigned from the company without further explanation, according to an exchange filing. The bank's shares are down nearly 16% this year.

5. Poon Ho Man, CEO of China Aircraft

CEO Poon Ho Man resigned with immediate effect in a letter received June 17 by China Aircraft's board. He had been on leave for a month, and gave no reasons for his decision to quit.

His resignation letter also made no reference to media reports linking him to a government investigation into China Southern Airlines, one of China Aircraft's customers.

"Except for news reported in the media, the Board does not have any information on the status of the alleged investigations, nor has the Board received any notice that Mr. Poon is under any kind of investigation," China Aircraft said.

The company also said it had not been able to contact Poon since receiving his resignation, and was "unable to verify the source of information of the news in the media." Company shares plunged 19% on June 19, and the stock has lost 33% so far this year.