In recent years artificial intelligence has been the hottest theme on stock markets. Companies building data centres and making chips rose at a dizzying pace. In the week of 12–16 October 2026, markets were reminded that what rises fast can also fall fast.

Korea took the biggest hit

On Thursday 15 October, Korea's Kospi index fell 7.6%, to 5,554 points. The fall was so steep that the Seoul exchange triggered its circuit breaker and halted trading for 20 minutes. It was the seventh losing day in a row – according to Korean media, the index lost about 20% in total over that period.

Tech companies, the backbone of the Korean economy, fell the most:

• SK Hynix, one of the biggest makers of memory for AI servers, fell 11%,
• Samsung Electronics lost 9.5%,
• foreign investors sold shares worth about $2.7 billion that day.

The same day Japan's Nikkei fell 4.9%. On Friday some of the losses were recovered: the Kospi gained about 2%, and foreign investors started buying again.

Why? Three reasons at once

1. Doubts about whether AI will pay off. Tech companies are spending hundreds of billions of dollars on data centres and chips, often with borrowed money. A warning sign came from CoreWeave, a US company that rents out computing power for AI: its shares fell 15.5% on Wednesday and its bond yields rose above 15%. Yields that high mean investors are worried whether the company can repay its debts. "Concerns over whether massive AI investment can generate sufficient returns had the biggest impact," said an analyst at Mirae Asset Securities.

2. US inflation is speeding up again. US prices rose 3.7% year on year in September (3.4% in August), and 0.6% month on month – the fastest since April. Producer prices jumped 6.3% year on year. That raises the chance the Federal Reserve will raise rates once more, and higher rates always hurt the valuations of growth companies.

3. Fear of a chip glut. Chinese memory maker CXMT is expanding production, which could push down memory chip prices and the profits of producers like Samsung and SK Hynix.

What about the US and Poland?

• The Nasdaq (the tech-heavy index) lost 2.4% over the week, the S&P 500 0.9%, while the Dow Jones – dominated by the "old economy" – actually gained 1%. The semiconductor index fell 4.3% on Wednesday alone.
• The 10-year US Treasury yield jumped to about 5.33% after the inflation data but fell to about 5.1% by the end of the week, as nervous investors moved money into safe bonds.
• The chance of a Fed hike as early as October fell below 15%; markets now expect it more in December. The next Fed meeting is on 27–28 October.
• In Poland things were calm: the WIG20 gained about 0.5% over the week, and the złoty strengthened against the dollar (USD/PLN below 3.88). The Warsaw exchange has few tech companies, so the AI sell-off barely touched it.

Is the bubble bursting?

Nobody knows today. One week of selling does not decide the end of a trend – in recent years AI stocks have dropped sharply several times and then gone back up. On the other hand, the questions about the profitability of AI investment and the debt of companies like CoreWeave are serious and will not disappear quickly. It is sensible to assume volatility will stay with us for a while.

What it means for you

• Do you have a fund or ETF on the "whole world" or the S&P 500? Tech companies make up a large share of them today, so you will feel the swings. If you invest long term and regularly, a week like this is noise – and when you buy during a dip, you buy cheaper.
• Do you have a lot in one theme (AI, chips, a single company)? This is a good moment to check whether you could live with a 20–30% drop. If not, consider spreading your money more widely.
• Not investing? The key lesson is universal: do not invest money you may need in the next few years, and do not buy something just because "everyone is making money on it".
• Remember the cost of living. High US inflation and expensive oil mean global interest rates may stay high for a long time. That is good news for savers in deposits and bonds, worse for borrowers.

In short

• Kospi: −7.6% in a single day, chipmakers about −10%, trading halted for 20 minutes.
• Reasons: doubts about returns on AI investment, higher US inflation, fears of a chip glut.
• US: Nasdaq −2.4% on the week, Dow Jones +1%. Poland: calm, WIG20 +0.5%.
• For long-term investors this is more noise than a reason to panic – but a good moment to check how exposed you are to a single theme.

Information and educational material, not investment advice. Data as of 16 October 2026.

Sources: Korea JoongAng Daily – Kospi plunges over 7%, KED Global – Kospi rebound, Analizy.pl – weekly market summary (16 Oct 2026). Photos: Unsplash.